When you created purchase order from a purchase requisition and later you deleted that line item in purchase order. As purchase order deleted, your purchase requisition for that line item will be closed and if your purchase requisition is open and you can also create purchase order from that open purchase requisition.
If you or your business needed, you could have gone for either one solution.
Option-1:
Before deleting the line item in purchase order, you could have click the “PReq Closed”(last column) in Delivery Schedule TAB in Item Detail of purchase order and then deleted line item. In this process in a single transaction (ME22N), you could have Closed purchase requisition quantity and Deleted purchase order.
Option-2:
As purchase order line item already deleted and you cannot do anything to that line item. But you could find your purchase requisition of that line item will be closed by selection of “Closed” indicator in Quantities/Dates TAB in Item Detail of the Purchase requisition. You can cross check report your purchase requisition is closed in t.code:ME5A by selection of check box Closed Requisitions.
OR
I’m not sure(how your system behaving), If purchase requisition is open after deleting the line item in purchase order and can close the line item in purchase requisition by selecting “Closed” indicator in Quantities/Dates TAB in Item Detail of the Purchase requisition and save.