Hi,
Alternatively, if have received the items from the Purchase Order, and therefore have a posted purchase receipt, you can pull receipt lines onto an unposted Purchase Invoice.
i.e., from the Purchase Order "Receive" the items:

This creates a posted purchase receipt (however the Purchase Invoice hasn't been created)
You can now pull that posted receipt onto an unposted Purchase Invoice:

You can then select the posted purchase receipt from the Purchase Order

The PO line is now on the unposted Purchase Invoice:

Hope this helps
Thanks
Gavin