Unfortunately a good answer was not obtained. We are told in Romans that none have obeyed the Law, for all have sinned. Is that what the disciple Stephen meant when he addressed the quoted passage to the Sanhedrin? If so, that raises the following problem: The Old Testament Israel never could have obeyed the Law, so that a) Stephen's remarks here are almost a redundancy and b) the failure of the Old Testament covenant was a certainty even before it had been given through Moses. Can this be true, but if not, what does Stephen mean in this passage with "law"?

Stephen was not exaggerating when he said that Israel had not obeyed it.

Stephen's words are not redundant. There were those among the Jews and therefore in his audience who believed they had kept the law. Even those who knew they had not kept the law needed to be reminded of the fact (as we do). It is true, as you say, that Israel never could have kept the Law. Even when the Law is reduced to the two "great commandments," LOVE GOD and LOVE YOUR NEIGHBOR, only one Person ever could and ever did keep the Law: Jesus Christ the Son of God and Savior of the world.

"What, then, was the purpose of the law? It was added because of transgression until the Seed to whom the prommise referred had come..." (Galatians 3:19). Why did God give a Law that no one can keep? "Through the law we become conscious of sin" (Romans 3:20), so that we know we need a savior.

Stephen is not talking here about whether Israel could keep the law, or what was certain before the law was ever given through Moses, or what kind of law they had broken. Recall James 2:10 - "For whoever keeps the whole law and yet stumbles at just one point guilty of breaking all of it." Stephen is calling them (and us) to repentance.