Subseq. entry of freight suppliers is not possible

What causes this issue?

During this goods movement, the 'Subsequent entry of freight supplier'
function was used.
Due to the way the system is configured, a delivery is created instead
of a material document. As a result, the system does not allow you to
change the freight supplier.

How to fix this error?

You can enter a different freight supplier by changing the purchase
order.

Error message extract from SAP system. Copyright SAP SE.