If possible, please use your email address as 'username'
Otherwise (user name too long etc): 'username' must be a single character string (no spaces) that is unique to this site. Allowed characters are letters, digits, at sign (@), period (.), plus sign (+), dash (-), and underscore (_).
If you enter a string that is already in use, you will receive an error message. If so, then change your username to taste.