Unfortunately the registration is not related to itch.io it's platform independent. (i.e. you'll have to register with the game, separate to the purchase of the game of itch.io)
I'll see if I can make that a bit clearer in game.
I can't see your account name registered. The email address has to have a normal format (i.e. xyz@something.com) and the password has to be a certain length. If you're still running into trouble and you're interested in trying to make sure you can register please reach out again and I'll fault find. Otherwise you can play the game without signing in. (it's just to capture leaderboard names for now).