You determined legs for a shipment which has the leg indicator
'subsequent leg'.
In general, such a subsequent leg shipment does not start at the actual
point of departure of the deliveries (for example, the ship-to
parties). It starts at a transportation connection point where the
goods are reloaded from a means of transport for the main leg shipment
to a means of transport for the subsequent leg shipment. For example,
this can be a port in which a ship is lying (the main leg shipment)
from which the goods are reloaded onto trucks (the subsequent leg
shipments). The trucks transport the goods to the customer.
The system determines this point of departure from the shipment number
you specified since at least one of the deliveries of the current
subsequent leg shipment is in the main leg shipment.

Error message extract from SAP system. Copyright SAP SE.