You can't have a portal without a relationship. In the relationship linking the layout's table occurrence (as selected in "Show Records From" in Layout setup) with the portal's table occurrence (as selected in "Show Related Records From" In Portal Setup), "Allow creation of records via this relationship" must be enabled for the portal's table occurrence or you will not be able to create new related records directly in the portal.
Normally, for a portal on a Purchase Order to list items that will be purchased on it, you need relationships like these:
PurchaseOrders::__pkPurchaseOrderID = POLineItems::_fkPurchaseOrderID
Products::__pkProductID = POLineItems::_fkProductID
If this notation is unfamiliar, see this thread for info on how to interpret it: Common Forum Relationship and Field Notations Explained
Your Portal would be set up to refer to POLIneItems in "Show Related Records From" and you'd set up _fkProductID with a value list of ProductIDs from the Products table.