If you're struggling to receive a PO, make sure the PO isn't in draft or received state. POs in draft state will have the Draft tag that looks like this:
Getting a PO out of draft state
To get a PO out of draft state, click into its detail page. Then, send the PO by clicking the Mark Sent button:
This tells the system that this PO has been sent to the vendor, and is now ready to be received.
From there, the PO will show up in the receiving list and let you receive items through the PO detail page receive buttons.