Unable to Get Mystery Champion

"An error has occurred

In order to unlock a Mystery Item your available items pool needs to be 10, and your current available items pool is 9."

This is the message I get whenever I try to unlock a Mystery Champion. I only have 5 things in my hextech items loot; 3 chests, a level 6 Ahri mastery token and an Asylum Shaco skin shard.

So first, I don't know why this can even be a thing with such a low number of items. I mean 10, really? Why would you limit someone to only having 10 things?

But then second of all, it must be bugged because I clearly don't have 9 items.