What causes this issue?

The number specified for the transportation service agent is a one-time
vendor, that is, a one-time service agent for which there is no master
record.
Name and address for this service agent have not been specified.

System Response

The system does not accept the service agent.

How to fix this error?

If you want to specify a service agent for which there is no master
record in the vendor master, you must enter the name and the address.

Error message extract from SAP system. Copyright SAP SE.