Adam Harishon did teshuva shleima [בראשית רבה פ' כ"ב] but it wasn't effective to absolve him from the punishment of death and to return him to Gan Eden. The Shlah writes:
When Adam sinned he became "חומר" physical matter and that could not be rectified by his teshuva. So it was necessary that he die in order to reach a state of purity.
OK. But why wasn't his teshuva enough?
There is a rule in creation: "בדרך שאדם רוצה לילך מוליכין אותו - A man goes in the direction he wants to go in". When a person chooses to live in a world of הסתר - hiddenness, that is what he gets. Adam wanted such a world. A world of נסיונות and difficulties where Hashem is not clearly visible. So that is what he got. Teshuva couldn't change that.
[עי' מכתב מאליהו ח"ה מעמ' 321]
