What causes this issue?

The fields employee group, employee subgroup and personnel area can be
changed only for special action types. The action type is changed to
values which does not allow any change to one of these fields.

System Response

The value of the fields are set back to the original-value which is
stored in the Personnal-Administration.

How to fix this error?

Use an action type which allows the change of the requested field.

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.