What causes this issue?

You can only set the <ZK>User Modification?</> flag if one of the
following conditions is met:
The <ZK>Display TMF on Execution</> flag is also set.
The <ZK>Validity Category</> field has one of the following values:
<ZK>Future-dated unscheduled</>
<ZK>Future-dated unscheduled correspondence</>

System Response

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

How to fix this error?

Make the changes according to the conditions listed above.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.