The purchase order is already invoiced. Forward exchange is not possible

What causes this issue?

You are trying to create purchase order subitems for forward exchange.
This is not allowed because the purchase order item is already invoiced.

System Response

The system does not allow further processing.

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.