I don't like his policies but I happen to think he's a generally decent guy, so this isn't meant as an attack on him personally, but rather to the point of his actions against the teachings of the church. And in case it goes down this route, you can make a million arguments about how Republicans are guilty of this too and I would agree.
With that said, abortion is the thing people most commonly point to. The Catholic Church is extremely anti-abortion and as I understand it, generally speaking Catholics who get an abortion are excommunicated. While Biden is male and thus not able to personally get an abortion, his policies and voting record (100% rating from NARAL) have expanded the ability to procure abortions which obviously goes against the teachings of the church, and not in a small or insignificant way.