You can use following steps for Finalization of PO .
Check the Status of the Purchase Order(PO) of Purchase Order is with approval status Confirmed & PO status ‘Invoiced’ than you can finalized directly but if its not invoiced then you can use below steps.
You need to do change request for workflow Click on ‘Request Change’.
Then Cancel the ‘Deliver Remainder’ for all the items in the Purchase Order.
Then Submit the Purchase Order for Workflow approval.
Then Make the Purchase Order ‘Approved’
Then Confirm the Purchase Order.
After above step now you can Finalize the Purchase Order.