It doesn't say that. Paul plainly says that all have sinned, Romans 3:10.
In Romans 3:20, Paul is explaining that Jews in the Old Testament were justified by obeying the law of circumcision, various external washings, various sin offerings, adherence to the dietary laws, and the observance of special days. They did not yet have Jesus to believe in, BUT NOW, NO ONE WILL BE JUSTIFIED FROM THE OLD TESTAMENT LAW. Paul is explaining that we must all go through Jesus now. Again, Jesus was not known by all in the Old Testament law, the law was not based on faith to be made clean before God, as we are by the law of faith in the New Testament, for Jesus' blood made us clean; the law in the Old Testament had certain works to do to make oneself clean before God. No one will be justified in that way anymore.
Romans 3:9 What shall we conclude then? Do we have any advantage? Not at all! For we have already made the charge that Jews and Gentiles alike are all under the power of sin. 10 As it is written:
“There is no one righteous, not even one;
11 there is no one who understands;
there is no one who seeks God.
12 All have turned away,
they have together become worthless;
there is no one who does good,
not even one.”
13 “Their throats are open graves;
their tongues practice deceit.”
“The poison of vipers is on their lips.”
14 “Their mouths are full of cursing and bitterness.”
15 “Their feet are swift to shed blood;
16 ruin and misery mark their ways,
17 and the way of peace they do not know.”
18 “There is no fear of God before their eyes.”
19 Now we know that whatever the law says, it says to those who are under the law, so that every mouth may be silenced and the whole world held accountable to God. 20 Therefore no one will be declared righteous in God’s sight by the works of the law; rather, through the law we become conscious of our sin.
Paul is quoting Psalm14:1-3; 53:1-3; and Ecclesiastes 7:20. Ecclesiastes SAYS there is not a RIGHTEOUS man who does what is right and NEVER SINS. Ecclesiastes tells us that there ARE RIGHTEOUS men but none who NEVER sin. Paul is explaining that the Jews were sinners just like the Gentiles. Nowhere in these passages or scriptures anywhere does it say none have to obey.
In this same Ecclesiastes 7, Solomon tells us about wise men, righteous men, and about a man who pleases God. Solomon also tells us that he found one upright man among a thousand. The Bible tells us that there are people who want to do right.
Now read on to Romans 3:21-26.
Righteousness Through Faith
Romans 3:21 But now a righteousness from God, apart from law, has been made known, to which the Law and the Prophets testify. 22 This righteousness from God comes through faith in Jesus Christ to all who believe. There is no difference, 23 for all have sinned and fall short of the glory of God, 24 and are justified freely by his grace through the redemption that came by Christ Jesus. 25 God presented him as a sacrifice of atonement, through faith in his blood. He did this to demonstrate his justice, because in his forbearance he had left the sins committed beforehand unpunished— 26 he did it to demonstrate his justice at the present time, so as to be just and the one who justifies those who have faith in Jesus.
In Romans 3:21,
we see that those who want to obey God--now is made known a way--faith in Jesus Christ. The way has been MADE KNOWN. The way has been MADE KNOWN. We have the choice to believe and obey God or reject Him.
The scriptures say that this righteousness from God comes through faith in Jesus Christ TO ALL WHO BELIEVE. This does NOT even imply that we believe just to believe, but we are to believe what Jesus says, and what does he say? He says to obey. THERE IS NO DIFFERENCE, FOR ALL HAVE SINNED AND FALL SHORT OF THE GLORY OF GOD...now all can be saved, Jew and Gentile, just by believing and obeying Jesus.