"Available items pool is 0": you've already got every Mystery reward
League clientStoreThe message
An error has occurred. In order to unlock a Mystery Item your available pool needs to be 1, and currently your available items pool is 0.What it means
You've collected every icon or ward the Essence Emporium's Mystery purchase can hand out, so there's nothing left for it to roll.
What to try first
Nothing to repair. Spend the essence on something else; the icons you're still missing come from a different reward pool.
Is it you or Riot? If lots of players are hitting this at once, it's probably the servers. Check League server status before you start reinstalling things.
You've emptied the pool
Granted, the wording sounds like a fault. Read the two numbers in it, though, and they give the game away: a Mystery Icon or Mystery Ward from the Essence Emporium rolls from its own set of rewards, and you already own all of them. With zero left to hand out, the store won't take your essence. Riot's article (updated May 29, 2026) gives no other cause, so there's nothing on your PC to fix.
Restarting won't help. Reinstalling won't either (a fresh client still sees the same full collection).
"But I'm still missing icons"
Riot's article was written with you in mind. An icon or ward you don't own can sit in some other reward pool that the Emporium's Mystery purchase never touches, so a few holes in your collection say nothing about what the Mystery roll has left.
Hextech Crafting is where the confusion usually starts. Its loot pools are separate from the Emporium's Mystery pools, so checking your collection against the Hextech list will show items you lack even after you've emptied the Mystery pool completely. Riot doesn't say where each missing piece comes from instead, only that it isn't this purchase.
Other wordings of the same error
League of Legends an error has occurred shopAn error has occurred Mystery IconAn error has occurred Mystery Wardyour available items pool is 0In order to unlock a Mystery Item your available pool needs to be 1
Related errors
Source: Riot Games Player Support, "An error has occurred" when purchasing Mystery Icons or Wards, updated May 29, 2026. Riot's steps are paraphrased here, not copied; checked September 29, 2026.