There are two possible causes of this error message:
Cause 1: Manager does not have System User access to the employee, Work Group etc in Portal. This is not the same as Org Role access.
Cause 2: The report is a WFM or Payroll report and Portal user not linked to a back office Username.
Resolution 1
Grant required Personnel access.
Resolution 2
Using Security, link the Manager's employee to the back office username.
The back office user's back office access will be used to display data on the report.