Partner &1 (&2) is not an employee

What causes this issue?

You may only use employees for assigning partner function &V2&. Partner
&V1& must be assigned to a central person.

Procedure for System Administrators

Procedure:
Create partner &V1&, if necessary as a business partner with the
function "Employee" (if nothing is provided from the employee records
to the HR system or if partner &V1& does not have any master data as an
employee).
Assign partner &V1& to the user. This can be done in the master data
under "Control" (under the "Employee function).

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

These activities may have to be carried out by the personnel
department.

Error message extract from SAP system. Copyright SAP SE.