Partner determination Issue

I run into a situation and need help from the forum. My goal is to determine a different BP based on a particular Buyer in sales order. To achieve this, I have configured partner determination for buyer that allows me to assign a BP to Buyer. As next step, when I try to assign buyer to BP in Source field (Partner Function Procedure). I get error "Partner function does not exist in the customer partner determtn procdr - Message no. VC267". Any suggestion is helpful. Thank you.