You can delete a purchase order that has been raised with a supplier as long as the order does not have any items marked as available or arrived.
How to delete a purchase order:
Use the search bar to look for the order number for the purchase order you want to delete.
Click on the purchase order from the list. You will be taken to the purchase order details page.
Click 'Quick links' in the top right corner.
Click Delete. Then type 'delete' into the box to confirm and click the 'delete' button.