In a sales order that contains both a service item and a product item

The new item I created is a service item. Therefore, I created a separate rule for the service item in Manage Process Assignment Rules for Sales Order and assigned it to DOO_BillOnlyGenericProcess. However, the test result shows: when the service item is ordered alone, no error occurs; but when the service item and a product item are ordered together, an error still occurs. Is this due to an Oracle system limitation, or is there another reason? If it is another reason, how can I resolve it?