1) Customer is on Hold
2) Sales line item is drop-ship
When you create a SOP/POP link in ver7.5, the system don't show you any message that the customer is on hold. When you try to generate the Purchase Order, the Purchase Order Generation Register report would show you an error- " The customer is missing".
This bug was fixed in ver8.0, now the Purchase Order Preview window would show you a message that the customer is on hold.