No tender possible for shipment & (forwarding agent missing)

What causes this issue?

The status 'write out' could not be set to 'Write out to shipper'
because no forwarding agent was entered.

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?

Enter the forwarding agent for the transfer and set the status
manually.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.