Make sure the S in your username is an uppercase one when logging in in the game. The website doesn't care, the game does.

@milliraryStuff55 (View Post)
You are probably typoing the username or the password. I'm almost certain of that.

The username needs to have two l's, "millirary", capital S "Stuff", "55".

If you still can't get it to work, you can try resetting your password: https://powdertoy.co.uk/PasswordReset.html

The case sensitive username is what causes most login failures though. So do make sure you have a capital S.