Resolution
Parallels Plesk Billing v6.0.0 is not compatible with Parallels Plesk Panel v9.0.1.Integration with Parallels Plesk Panel v9.0.1 is implemented for Parallels Plesk Billing v6.0.1.
If you upgraded Parallels Plesk Panel to v9.0.1 you need to upgrade Parallels Plesk Billing to v6.0.1.