If you're getting the Shadow of Erdtree unable to summon cooperator message, then you're stuck trying to summon help and, short answer, the only real option right now is to keep trying.
There's been no official confirmation of the problem yet, or mention of a fix, and that's probably because there's not actually anything wrong, it's just a question of supply and demand.
How to fix the Unable to Summon Cooperator issue in Shadow of the Erdtree
The problem with the inability to summon a cooperator in Shadow of the Erdtree is that nothing seems to be technically broken. It's more that the demand for help is far outweighing the demand, with the problem mainly manifesting in high traffic areas like just before the Erdtree Rellana Twin Moon Knight boss fight.
What appears to be the issue is that so many people are looking for help in Shadow of the Erdtree that all the summons you see are claimed within seconds, long before the symbol appears in your game. So, you're unable to summon because someone else got it first.
The unable to summon a cooperator in Shadow of the Erdtree error fix, from my own experience, is to just hoover up all the summons as fast you can. You'll get the unable to summon message repeatedly but just keep going. You'll probably clear the floor of symbols but then a new batch will appear a few seconds later. Try to clear them again as fast as possible and, if you're lucky, you'll finally get the summoning cooperator message. I was stuck outside Rellana for a while and this method always got me a summon, usually on the second wave of spawn.
You've basically got to be quick - if you just sort of summon, wait for the fail message, summon again and so on, you'll never get anywhere as potential cooperators will be stolen by the massive volume of players looking for help. The downside is that you can't really afford to be choosy and don't really have much control over you get, but you will get someone. Eventually.
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