An error has occurred.

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

1. My available champion pool needs to be >= 10 or > 10?
2. Clearly, 3 × 3 - 1 + 2 = 10. Is there some untold information I should be aware of?

Reddit Megathread

Update : Support Ticket

SelfAwareNPC

November 25th 2017, 4:24:59 pm

Hello Nato Boram, This isn't an issue that comes up a lot, but if you own most of the champions in the game, then there isn't much of a selection left for the Mystery Champion pool. An account has to have at least 10 un-owned champions in order to purchase a Mystery Champion in order to prevent the game from accidentally granting a duplicate. If you do have 10 un-owned champions and any of them are currently on sale, the system won't let you purchase the Mystery item either. The system flags it because you could potentially buy the Mystery Champion for more points and end up with a champion you could have just bought at a lower price. If one of your un-owned champs in Zoe, she may not have been added to the Mystery Pool yet--that can take a couple days, and if you check back in a week or so it should be updated.

I hope this clears up any confusion, and do let me know if you have any other questions :)

SelfAwareNPC Riot Games Player Support

Basically, this bug exists for financial reasons.