Elden Ring is a well-famed game that has fans all over the globe. While playing the co-op, players have complained about the “Elden Ring unable to summon cooperator” quite often.
The error is likely to appear when someone else has summoned a similar operator before you. However, if this is not the case then there are other reasons as well causing the problem.
So, if you are also seeing the “Unable to summon cooperator, failed to join session error” then follow the below-given fixes one by one till you find the one that works for you.