[CLIENT] Can't create summoner name

I just created an account and I can't give it a summoner name. When I type in an available name and press 'Start Now' it just says "An unexpected error occurred. Please try again." I've tried reopening the client and repairing it twice, but it won't help.

Really frustrating because I wanted to create a new account after not playing League for a couple months for a fresh start but I can't.