One connection, several carriers
Navlungo is not a single carrier but a broker that manages several carriers from one panel. So setup includes a carrier choice, made with numeric codes. On the default automatic setting the shipment is routed to a suitable carrier by coverage area; if you want to work with one specific company you enter its code.
Creating shipments also requires a sender address ID, which you take from the record defined as a sender in Navlungo's address book. If there is no sender record in the address book, shipment creation will not work — this is the prerequisite most often skipped during setup.