You must have signed up for Playstation network after you formatted using a different PSN account. Once you sign in with a PS3 user profile, you always have to sign-in with that same PSN account. If you try to sign-in with a different PSN account, it will fail, and you will get that message. You need to either create another PS3 User login profile to use with this account, or use the Restore Default Settings command in System Settings to clear the PSN login information from the user profile that you already made. Usually the first one is the best choice.