Driver / Material Picker mandatory in Delivery

I have created two business partners Driver & Material Picker (partner type PE) and I have assigned to Delivery Header. My intention is to throw an error when the Partners are not filled. Even though in the incompleteness, it shows that the Business Partners are not filled, the system allows me to save the delivery. I have assigned the status group G2 (all checked in Dly/Billing etc), but I am unable to get the error message. Can somebody help me in stopping the delivery not to be created without the business partners?