There are already follow-up documents. 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 follow-up documents already exist. This may
be a delivery or (uncancelled) material movements.

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.