God cannot sin because God is all-wise and all-knowing. An all knowing and wise person will not choose to sin, because to do so would be foolish. Man sins because his knowledge and understanding are limited. He needs to learn that the consequences of selfishness will be destructive to himself and others.
You are fallaciously presupposing that man being "not good" means man is 100% evil and therefore incapable of good". "Not good" means not 100% good, like an buyer considers an apple "not good" if it is flawed; but it may still have many good parts that can produce good juice when squeezed.
There is not one perfectly wise human, therefore there is no human, who relies at times on their own wisdom, who always does good, not one. There is no human, who relies at times on their own wisdom, who is always righteous, not one. And yet, evil people, from their good parts, can do good. "You who are evil know how to give good gifts to your children." Mt. 7:11