What partner function could cause a system message indicating a missing business partner after saving a sales order?

The sold-to party is crucial in a sales order as it represents the customer who is purchasing the goods or services. This partner function is essential for generating invoices and is tied directly to the financial aspects of the sales document. If the system identifies that the sold-to party is missing or not correctly assigned when saving a sales order, it will trigger a system message indicating a missing business partner. This occurs because the sold-to party's information is vital for processing the order, including determining pricing, delivery, and payment terms.

In contrast, while other partner functions like the bill-to party, deliver-to party, and ship-to party are also important for specific processes within sales order management, they do not impact the basic requirement of having a sold-to party identified in the system. Therefore, their absence might not lead to the same type of critical error message upon saving the sales order as that of the sold-to party.