PO already created, order change is not possible

What causes this issue?

Purchase Order created, order change is not possible

System Response

A&D Subcontracting is updating Purchase Requisition from the
corresponding PM/CS Order.
If the Purchase Order created from the Purchase Requistion then furter
changes are
not allowed in the relevant operation/component level because these
changes are
not reflected in the PO after its creation and can lead inconsistencies.

How to fix this error?

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

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.