First, check if you can log in on the forums. If you can't, do a password reset: https://powdertoy.co.uk/PasswordReset.html

If you can log on in the forums but not in-game, then make sure you are capitalizing your username properly. It is case-sensitive, but only in game (until 96.0's release).

Edit: actually, you need to verify the account. Please check your email for the verification link. Your account isn't active until it's verified.