Unable to assign performance document to a person, getting some error message

While assigning performance document to an employee from My Client Groups>Performance>Performance Document>Create Document, I am unable to add Performance document for that employee, getting below error message :

"We couldn't find any matching eligible employees. Employees without performance documents already created should not only be eligible, but also have a manager, and their assignment dates must partly cover the performance document dates"

Can anyone suggest on this please?

Thank You.