Diagnosis
You have defined that thepartner function "Ship-ToParty" does not have to be a mandatory partner for deliveries. However,this function is a mandatorypartner function when you are working with outbound deliveries anddeliveries for returns.

System Response
The system issues a warning. If the ship-to party is not defined as amandatory partner function, processing of outbound deliveries andreturns deliveries for customers ends up being terminated.

Procedure
Check whether the current determination procedure is assigned to adelivery type for outbound deliveries (SD document category 'J') or returns deliveries for the customer (SD document category'T'). Check the flag "Partner Is Mandatory" if this is the case.
You can use the partner function "Ship-To Party" as an optional role forinbound deliveries.