Intra-Company Stock Transfer PO is requiring vendor inv to bill customer

I am trying to configure a process where a new item category on a sales order will trigger a purchase requisition (new req type) to create a UB purchase order. When I deliver the PO from the supplying plant, then do the goods receipt in the receiving plant, then deliver and PGI the delivery for the sales order, I cannot invoice the customer because I get the error message 0141 Vendor Invoice has not yet been created (Individual Purchase Order). I don't think I should need to create a vendor invoice for a stock transfer between 2 plants in the same company code, and when I try I get a message that no suitable item is found for the purchase order.

Can any of you experts tell me what determines whether or not a vendor invoice is required, and why I'm getting the message that one is required?

REGARDS,