If you have converted a purchase order, it creates an associated bill for that purchase. In order to delete the purchase order you first need delete the associated bill.
*Note - for future reference, you should not convert a purchase order until you have received the bill from the supplier.
To delete a converted purchase order:
Step 1 - Go to bills and search/find the associated bill (you can search via type "Purchase invoice")
Step 2 - If the bill has been confirmed, first release the bill and then delete
Step 3 - Go back into Purchases and you can now release and delete or edit the purchase order
Here is a link to the HELP file - https://login.shoebooks.com.au/net/pages/Help.aspx?from=PurchaseEdit