Account determination error while doing Invoice

Hello all,

I'm having an error while posting an invoice for a PO.

I created a PO for a non-valuated item and selected "non-valuated GR" checkbox.
Inside OBYC, there isn't anything that is maintained for WRX.
For GBB, there is a valuation class and an account that is mapped, and this is picked while creating the PO.

Why can't the system post an invoice? If non-valuated GR checkbox is checked (as I did), why does it need WRX (GR/IR posting)? Shouldn't the setting in GBB be enough?

Please help
Thanks