Sage 500

I'm trying to close lines on a open purchase order and it says I can't. I tried using the Receipt of Invoice and it will not pull over that purchase order. Does anyone have any ideas?

In a lot of cases we prepay our vendors for all or a portion of the goods. The invoice is received and partial or all of the payment is made prior to the goods being shipped. We use purchase orders within the inventory process. How do I put in the invoice…