Yes, you need to reach a higher Summoner Level in order for your allowed Cat Inventory to hold a higher number of Cats to use for Rankups (without overflowing into Stash).Real problem seems to be the increased widespread availability to get 5* and 6* champs nowadays and their high level Cats without actually having played the game long enough yet.I assume this is a 2nd (alternate) account, otherwise why haven’t you been able to get higher than Level 36 on it yet if playing for almost 2.5 years already.