Queries aren't immediately consistent, so an entity that you've only just created won't be returned in a query performed right-away, you need to wait a bit.
In your case, you don't need to query for the entity - you just created it, so you know it exists, and can use it. Change your create_game function to return the new game, and then use that.
If you expect your user_id to be unique (and given your query, this seems to be the case) you could use it as the entity-id, then you can get-by-id instead of querying, which will strongly-consistent.
Check the "Data Consistency" section on this docs page for more detail on how queries work.