Forward exchange not possible: item &1/&2 already has subitems

What causes this issue?

You are trying to create subitems for forward exchange in a purchase
order. This purchase order already has subitems.

System Response

The system did not create forward exchange subitems for the purchase
order.

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.