Referenced reservation was not created manually

What causes this issue?

The reservation you entered as reference was not created manually.

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?

You can only copy reservations that have been entered manually.
Enter a different reservation as reference or create the new
reservation without referencing an existing one.
Press F4 on the field "Reservation" to search for reservations for a
material or an account assignment.

Error message extract from SAP system. Copyright SAP SE.