You determined legs for a shipment which has the leg indicator
'preliminary leg'.
In general, such a preliminary leg shipment does not end at the actual
destination point of the deliveries (for example, the ship-to parties).
It ends at a transportation connection point where the goods are
reloaded on a means of transport for the main leg shipment (for
example, the port where the goods are reloaded onto a ship).
The system determines this destination point from the shipment number
you specified since at least one of the deliveries of the current
preliminary leg shipment is in the main leg shipment.

Error message extract from SAP system. Copyright SAP SE.