A system user, which was created during a new subscription setup, cannot log in to Plesk.
Client is trying to access Plesk with subscription user credentials.
Log in to Plesk using customer, reseller or additional users credentials, since by design only they can access to Plesk.
Additional user can be created via Home > Subscriptions > domain.tld > User > Create User Account. This user can be used to access Plesk to manage the subscription, depending on permissions granted to him.
Note: a system user of a subscription is able to log in to Plesk, if the system user and the corresponding customer have the same name.