Without more information it's impossible to say why you couldn't join your friends league. We could have investigated it, but you already joined a different league. All of your clubslots are occupied.
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If you still want to join your friends league, you will have to resign on any of your clubslots.
Because you already joined a different league and gave no concrete information, I will close your topic. If it happens again in the future, then make some screenshots of the issue and report it again.