Galatians 3:21. “Is the law then against the promises of God? God forbid: for if there had been a law given which could have given life, verily righteousness should have been by the law.”
If law could bring righteousness and life, then justification would be by the law. But the scriptures of the prophets as well as the law declare all to be sinners (Rom. 11:32), imprisoned under sentence of death until the sentence should be carried out upon those sinners; that the promise of life by faith might be given to those who believe when Christ paid their penalty (vv. 22-23).
The law did NOT replace the promise. It only taught us how to live until the promise was fulfilled in the Person of the spotless Lamb of God, Who would fulfill the law and provide salvation BY FAITH, not by the works of the law. There is no law that could give life, eternal life, to anyone. If there were, then righteousness would have been by the law, and not by grace.
Paul seems to be saying the same things over and over, doesn’t he? That’s because he is carefully building an unassailable case against salvation by works. Every detail had to be covered, every possibility explored.