Thanks for posting in the Community. I'd be glad to help you delete a PO in QuickBooks Online.
There are times when a transaction, such as a purchase order, will need to be deleted from a QuickBooks Online account. Here are the steps to delete a purchase order:
Open up the purchase order.
Select More at the bottom of the screen and choose Delete.
Click Yes that you want you to delete the transaction.
I've included a brief video to serve as a demonstration: