What does the Unable to Summon a Cooperator error mean in Elden Ring, and how to fix it

gamepur.com:

If you’ve ever participated in jolly cooperation in a Souls game, you know that the connection between players can sometimes be the opposite of stellar. Undoubtedly, if you’ve tried to summon players to help you against a boss, you’ve come across an error message that reads “Unable to summon a cooperator.” If you’re curious what this means and how to fix it, read on.

The “Unable to summon a cooperator” error means one of two things — the connection between you two is unstable, or more likely, the player was summoned by someone else first. The type of multiplayer the Souls series is known for relies on what is known as “Peer-to-peer” connections — that is, one player “hosts” the other players in their world. This means that a bad connection from the host player, or a disconnect from the host to one of the phantom players, can result in some weird game behavior.

Inversely, if someone is summoned elsewhere before you get to them, their sign will not disappear in your world right away. The game will try to call them to your world, recognize that they are already in another world, and spit back the “Unable to summon a cooperator” error at you. This is extremely prevalent right before boss doors, as most players that drop signs in those locations are attempting to farm the boss for Runes.

However, if you fail to summon several people in a row, you may wish to restart your game client and refresh your connection to the Elden Ring servers. This is the easiest and quickest way to fix any potential problems with connecting to the game servers that you might be having.