Check the Club to see if you are assigned as the Owner instead of just a member or admin.

Club Owners cannot leave a club while other members are still in it. You must assign the Club Ownership to another member in the Club first before you can leave. Tap on the blue gear icon next to the name of a member in the Club list to assign them as the Owner.

As the Owner, if you are the last one to leave the Club, leaving it will also delete the Club from the game and there will be no way to retrieve or rejoin the Club as no members exist. The Cherries that were spent to create the Club will not be refunded.