SymptomsPanel throws the following error on attempt to login:
Error: You entered credentials of the Panel user account that is associated with a billing account. For security sake, the system cannot allow you in under the Panel user. Please use your billing account username and password to log in. You can look up your billing account login information in the message e-mailed to you when your subscription was created.
CauseIf you create a customer in the Billing system, the credentials, access details specified during this process are to be used to login the entire system. This is required as these access details are added to the Single Sign-On (SSO) server database thus you are allowed to switch between the Panels without entering them again.
If you have a client in the Plesk Panel (PP) only, you can login to the PP only using the credentials specified for this user.