If you mistype your email address when signing up (or discover that your email address doesn't accept mail from userscripts.org) then the account is useless and can't be deleted or changed, which means you cannot re-use the display name for a new account. It ought to be possible to delete the old account (either using the password or automatically after it has been dormant for a few days) or to correct the typo in the email address so that it can be re-used.