Freight supplier of purchase order is not the invoicing party

What causes this issue?

Planned delivery costs have been entered in the purchase order. A
different "freight" supplier has been specified for the delivery costs
which have not yet been invoiced. Therefore, delivery costs cannot be
posted for the supplier for whom you are currently entering an invoice.

How to fix this error?

Using the function <LS>Display purchase order</>, you can determine for
which supplier the delivery costs have been specified.
If you want to post the delivery costs in this invoice, you can only
enter them as unplanned delivery costs.

Error message extract from SAP system. Copyright SAP SE.