A Draft and Approved purchase order can be deleted before it is at the Received stage using the delete buttons on the PO detail page.

Once a PO has been received, you can no longer delete it as that is the stage it has locked in the stock movement in the background. It is extremely important that the PO is only received if the stock has been checked with what has actually been delivered and/or cross checked with the supplier invoice.

To resolve this you can also receive the PO and change the received quantity to 0 so the stock movement is not affected.