Once you have agreed all the details of a purchase order with your supplier, it's time to finalise the PO, in preparation for receipt of the inventory

In order to do this, all you have to do it open the PO and click "Mark as pending"

And then confirm from the pop-up you do want to mark the PO as pending. 

Once you've marked a PO as pending, you can no longer make any edits. If you need to make changes, you should duplicate the PO and then 

How to finalise a purchase order