This only happens when the server is under a larger than normal amount of stress, which usually happens when a new champion is released (and especially now with Lulu on the PBE). To prevent too large an amount of custom games, it requires a larger number of people (usually two) before you can start one. Try again when it's not peak time in your region and it'll work fine with just one player again. You have to keep in mind that even if you play solo in a custom game, it's still a full game of