Leg determination has not been carried out (no destination)

You tried to determine legs for a shipment which has the leg indicator
'preliminary leg'.
Usually, this kind of preliminary leg shipment does not end at the
actual destination of the deliveries (for example, the ship-to
parties). It ends at a transportation connection point where the goods
are reloaded onto a means of transport for the main leg shipment (for
example, the port where the goods are loaded onto a ship).
You should either specify this destination manually (only possible in
the 'create shipment' (VT01) or 'change shipment' (VT02) transactions,
because at least one of the deliveries of the current preliminary leg
shipment is in a main leg shipment.
The target destination was not entered manually and could not be
determined by the system so the legs were not determined.

Error message extract from SAP system. Copyright SAP SE.