Error - "Could not determine the cost center for employee".

Hello SAP S/4HANA Cloud Community

I work in Product Support for S4HANA Cloud and I'd like to share questions I receive through incidents with the Cloud Community as I think it would be beneficial to share the information

Question:

An error "Could not determine the cost center for employee" occurred , when trying to staff an employee to a workpackage in a customer project in "Plan Customer Project" app.