This issue is actually a server-related problem.
There are currently workarounds for this since Blizzard has not made any official fixes yet. It is usually with this message: This character is already in a game on the server. Some players have encountered the Failed to Enter game issue recently when playing co-op with friends.
Diablo 2 Resurrected This Game is No Longer Available to Join Issue Fix So here is how to fix the Diablo 2 Resurrected ‘doesn’t start’ and ‘failed to enter game’ issues until the developers find a permanent solution.
There have been some who have experienced issues like the game “doesn’t start” and “failed to enter game” and unfortunately there is still no fix from Blizzard. Diablo 2 Resurrected just launched and already some players are having some problems with it.