Carriers in freight order and freight settlement document do not match

What causes this issue?

You have attempted to add a freight order to a freight settlement
document, which you are creating manually. The carrier in the freight
order is different to the carrier in the freight settlement document.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

You can only add a freight order that contains the same carrier as the
freight settlement document.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.