Two Things!
I do not reject the Grace of God, for if righteousness is by the written law, the Messiah died for nothing!           Aramaic in plain English, (Jubilee Bible 2000)

I do not set aside the grace of God; for if righteousness is through Law, then Christ died without cause.                                    J. Green Interlinear Bible, KJ3

Here is how other versions of the Scriptures translate Galatians 2:21

I do not set aside the Grace of God;             NIV, BSB, BLB, DBT, HCSB, NET BIBLE
I do not treat the Grace of God as meaningless;                                               NLT, WNT
I do not nullify the Grace of God;                                                ESV, NASB, NAS 1977
I do not frustrate the Grace of God;              KJV, LAKJV, WEBSTERS BIBLE T., YLT
I do not misapply the Grace of God;                                                                            ISV
I do not make void the Grace of God;                                                     ASV, ERV, WEB
I cast not away the Grace of God;                                        DOUMAY-RHEIMS BIBLE

The Greek word is #114 in Strong’s concordance; atheteó: to do away with what has been laid down, set aside
Original Word: ἀθετέω, Part of Speech: Verb, Transliteration: atheteó, Phonetic Spelling: (ath-et-eh'-o)
Short Definition: I annul, Definition: I annul, make of no effect, set aside, ignore, slight; I break faith with.
114 athetéō (literally, a-thetos, "un-place") – properly, do away with; reject what is already laid down; to set aside (disregard as spurious); nullify, make void; to break faith (Abbott-Smith); remove out of an appointed (proper) place, i.e. reject as invalid; refuse to respect (even "despise"); to cancel, disannul, abrogate (passive, "be set aside" because perceived to lack value); to disregard, pass over (refuse to acknowledge).

This Greek word (atheteo) is used 16 times in the New Testament.
Mark 6:26 and having become deeply grieved, but because of the oaths and those reclining together, the king did not wish to REJECT her.
Mark 7:9 And He said to them, Well do you to SET ASIDE the commandment of God so that you may keep your tradition?
Luke 7:30 But the Pharisees and the lawyers SET ASIDE God's counsel as to themselves, not being baptized by him.
Luke 10:16 (4) The one hearing you hears Me, and the one REJECTING you also REJECTS Me, and the one REJECTING Me also REJECTS the One having sent Me.
John 12:48 the one who REJECTS me and does not receive My Words has that judging him: the Word which I spoke, that will judge him in the last Day.
1 Corinthians 1:19 for it has been written, "I will destroy the wisdom of the wise, and I will SET ASIDE the understanding of the understanding ones." Isa. 29:14
Galatians 2:21 I do not SET ASIDE the grace of God; for if righteousness is through Law, then Christ died without cause.
Galatians 3:15 Brothers, I speak according to man, a covenant having been ratified, even among mankind, no one SETS ASIDE or adds to it.
1 Thessalonians 4:8 (2) Therefore, the one that DESPISES does not DESPISE man, but God, even He giving His Holy Spirit to us.
1 Timothy 5:12 having guilt because they SET ASIDE the first faith;
Hebrews 10:28 If anyone did not REGARD the Law of Moses, that one dies without pities on "the word of two or three witnesses," Deut. 17:6
Jude 1:8 Likewise, indeed, also these dreaming ones even defile flesh, and DESPISE rulership, and speak evil of glories.

                      One way of salvation   
God has arranged one way for us to be saved and only one way. That one way is through his Son Jesus Christ. (John 14:6, John 6:37, 1 John 5:5, Acts 4:12) Jesus death on the cross for our sins is the only acceptable sacrifice. God applies what his Son has done for us to us by his Grace. There is no other way. Salvation is by the Grace of God. None of us can enter heaven the way we are, for we all have sinned and therefore fall way short. We cannot make up for our sins and God will not “forget” or “overlook” our sins. It is impossible to earn our salvation. Even the “best” person in this world and the most do gooders do not deserve heaven because we all sin. Good deeds cannot make up for our sins, nothing can make up for our sins. The punishment for sin is eternal death. Jesus dying on the cross for our sins is the only acceptable sacrifice for our sins. Anyone trying to earn their salvation or counting on their good deeds to be acceptable to God is rejecting Jesus sacrifice for us. Even “mixing” grace and works/efforts or living by the law is rejecting God’s way and it will not work anyway. It is either God’s way or no way, there is no in between. God gave up his own son for us, for his own to thankfully accept this amazing grace gift, living thankfully for God’s grace. Trying to earn our salvation or work for it or any returning to living by the law for our righteousness greatly displeases God and is in effect saying; “Christ died for nothing.”
The best way of saying the first part of Galatians 2:21 may be;
I gladly accept (I don’t reject) the way God has arranged my Salvation; Salvation is by his Grace and his Grace is the only way for Salvation, and I am thankful.

                        The 2nd thing;
if righteousness is through the law, then Christ died for no purpose. Only the Righteous will be able to stand before God on the last day. Being Righteous and becoming Righteous is very important. Jesus told the people that unless their Righteousness exceeded that of the Pharisees and Sadducees they would never enter the kingdom of God. (Matthew 5:20)The law or living by the law cannot make anyone righteous.
Here are the righteousness verses that are just alike this one;
Matthew 5:20 For I say to you, If your righteousness shall not exceed that of the scribes and Pharisees, you shall not enter into the kingdom of God, never!
Luke 1:68-75 Blessed be the Lord, the God of Israel, because He visited and worked redemption for His people. And He raised up a Horn of salvation for us in the house of His servant David; even as He spoke through the mouth of His holy prophets from the age before: Salvation from ones hostile to us, and from the hand of all the ones hostile to us, to execute mercy with our fathers, and to remember His holy covenant, the oath which He swore to our father Abraham to give to us, that we being delivered out of the hand of those hostile to us, in order to serve Him without fear, in consecration and righteousness before Him all the days of our life.
Romans 1:16&17 For I am not ashamed of the gospel of Christ, for it is the power of God to salvation to everyone believing, both to Jew first, and to Greek; for in it the righteousness of God is revealed from faith to faith; even as it has been written, "But the just shall live by faith." (Hab. 2:4)
Romans 3:21&22 But now a righteousness of God has been revealed apart from Law, being witnessed by the Law and the Prophets, even the righteousness of God through faith of Jesus Christ toward all and upon all those believing; for there is no difference,
Romans 6:18-20 And having been set free from sin, you were enslaved to righteousness. I speak as a man on account of the weakness of your flesh. For as you presented your members as slaves to uncleanness and to lawless act unto lawless act, so now yield your members as slaves to righteousness unto sanctification. For when you were slaves of sin, you were free as to righteousness.
Romans 10:3 For being ignorant of the righteousness of God, and seeking to establish their own righteousness, they did not submit to the righteousness of God.
1 Corinthians 1:30 But of Him, you are in Christ Jesus, who was made to us wisdom from God, both righteousness and sanctification and redemption,
2 Corinthians 5:21 For He made the One who knew no sin to be sin for us, that we might become the righteousness of God in Him.
Galatians 2:21
Galatians 3:20&21 Then is the Law against the promises of God? Let it not be! For if a law had been given which had been able to make alive, indeed righteousness would have been out of Law. But the Scripture locked up all under sin, that the promise by faith of Jesus Christ might be given to the ones believing.
Ephesians 4:24 and to put on the new man, which according to God was created in righteousness and true holiness.
Titus 3:4&5 But when the kindness and love of God our Savior toward man appeared, not by works in righteousness which we had done, but according to His mercy, He saved us through the washing of regeneration and renewal of the Holy Spirit,
1 Peter 2:24 who "Himself carried up in His body our sins" onto the tree; that dying to sins, we might live to righteousness, of whom "by His wound you were healed."
2 Peter 1:1 Simon Peter, a slave and apostle of Jesus Christ, to those equally precious with us, having obtained faith in the righteousness of our God and our Savior, Jesus Christ:

On the last day, at the last judgement God and Jesus Christ will separate the righteous from the unrighteous, those who had been made righteous through Christ’s sacrifice and those who had only the Pharisees outward righteousness, but inwardly were full of their sins. The righteousness given to us by God causes thankfulness and humbleness because we know what we deserve and we know that we could never have obtained that righteousness of ourselves. The righteousness of the Pharisees causes pride; look at me, look how good I am, how well I serve God and how “holy” I appear.
If I could be righteous by keeping the law then Christ died for nothing. Even if I try to obtain righteousness through the law, what I really am saying that is that I don’t need Jesus’ death on the cross! When it comes to righteousness and justification the law can only condemn. The law or keeping the law cannot justify anyone, nor can it make anyone righteous. The law cannot make up for our sins nor can it take them away, matter of fact the law only increases sin, and makes me sinful beyond measure. If you try any other way but glady accept the free gift of righteousness through our Lord and Savior Jesus Christ you will fail and really you are opposing God and making a mockery out of what Jesus did for us on the cross by saying that really, it wasn’t necessary. Only the righteous will be able to stand before God, those who have a righteousness bought by our Lord and Savior Jesus Christ, a righteousness of faith. They will be able to stand before God; meaning they won’t be shrinking back in shame nor will not they hear; depart from me, I never knew you. All because they were made righteous through Jesus Christ’s great sacrifice!
After anyone comes to Jesus Christ and becomes a new creation, they are very thankful. But sooner or later the enemy and the flesh try to creep back in and try to gain the upper hand again by suggesting that “you’ve got to do something”, or that in order to be religious they must do such and such things. Don’t get lured back into slavery, because it is for Freedom that Christ has set us free, therefore do not again submit to that slavery yoke. Instead, remain thankful, remember what Jesus Christ has done for you that you are saved by his grace through faith and let anything you do come from that, being led by his Spirit into the works he has prepared for you beforehand!

Here is what Martin Luther said;
From whom then do we obtain the thoughts that truly excuse us? Only from Christ and in Christ; for if the conscience of a believer in Christ reproves, accuses and condemns him as an evil doer, he quickly turns from himself to Christ and says; "He has atoned for my sins. He is just and my justifier, who died for me. He has made his righteousness my own and my sins his own. But if he has made my sins his own, then I no longer have them but am free from them. And if he has made his righteousness my own righteousness, then I am righteous because of his righteousness, for he is God, blessed forever!​​​