Purchase requis.&1 item &2 can no longer be changed

What causes this issue?

You have changed a schedule line for which the quantity must be
procured externally. The quantity is procured using a purchase
requisition which refers to the schedule line. The purchase requisition
has a status which limits the possibilities of changing.

System Response

The purchase requisition is not changed automatically when the sales
order is changed.

How to fix this error?

You can change the purchase requisition manually from the schedule line
screen. If you cannot make the changes there, you must inform the
purchasing department.

Error message extract from SAP system. Copyright SAP SE.