What causes this issue?

The selected relationship is not valid. Valid relationships are those
defined in the following Customizing path: <ZK>Personnel Management</>
-> <ZK>Organizational Management</> -> <ZK>Basic Settings</> -> <ZK>Data
Model Enhancement</> -> <ZK>Relationship Maintenance</> -> <ZK>Maintain
Relationships</>.

System Response

The system does not permit this selected relationship.

How to fix this error?

Select a valid relationship.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.