The error will read something like below:
The DELETE statement conflicted with the REFERENCE constraint "FK_purchase_order_item_purchase_order_id". The conflict occurred in database "ServiceLedger", table "dbo.purchase_order_item", column 'purchase_order_id'
This error is the result of trying to delete a purchase order that has items on the purchase order. You must remove the items first before you can delete the purchase order.
Note: If you have received any items against a purchase order, you will not be able to delete the purchase order.