Why was the man who copied his neighbor and brought a sacrifice not accepted?
He was not accepted because he brought the sacrifice merely by copying his neighbor, not by faith. Though the outward act matched God's instruction, his heart did not act because God had instructed it, so it was not done according to faith.
Les explains that the man was not accepted even though he brought a sacrifice, because he did it simply because his neighbor had done the same thing. He did not do it by faith, and he did not do it because this is what God instructed to do. Les contrasts this with the man who admitted his sin but refused to bring the required sacrifice to the priest; that man also was not accepted because he did not do what God said to do. The point is that a person can go through all the motions, but if he does not do it explicitly according to God's instructions, he will not be accepted because he did not do it according to faith. On the other hand, if he has faith but does not carry out the work, he is not accepted either.