SAP Message 9J744 - Change to measurement document &1 not possible currently

Diagnosis
You cannot currently change the measurement document for a subitem.

System Response
You can only change the measurement document if the subitem quantity forthis measurement document is ready for input. This is not the case atthe moment.

Procedure
If required, change the external document number in the header data areaso that it is identical to the external document number of theappropriate item.