Coop Issues

I play on PS4 and I can't cooperate with my friend who is on PS5, why?

You definitely can.

There are a large number of reasons you might have difficulty:

1) The two of you are not playing in the same league (such as one is Hardcore and one is not, or one playing the current league, Affliction, while the other is playing "standard", etc).

2) One of you has some PSN settings preventing the grouping

3) One of you has blocked the other in-game

4) One or both of you currently has "the group bug", which prevents partying at all, but can be (temporarily) fixed by restarting the game. You can identify the group bug because the corner of the screen with name (while at the options menu) will not show (1/6) like it usually does.

5) You may simply be going about the grouping wrong, for example trying to use the Playstation system "join game" feature, which can be wonky between PS4/PS5 (but usually works fine on the same platform). Instead join up either through a PSN party or using in-game features.

There are probably more, but these are the possibilities that came to mind.