* The location of the PO must match the location of the warehouse
* The PO can't be opened anywhere on the network by any user
* No part of the PO can be locked by any user. (I suspect this might only arise if there was some attempt to shipment cost the PO, even perhaps by mistake.)
Neither scenario should be the issue if you're able to edit and resave the PO.
* The status of the PO must either be set to 'partial' or 'on order'.
* The quantity on each line item ordered must be greater than the quantity already picked.
In the above case use Data Navigator to check if SOSTOCK->QTY > SOSTOCK->QTYP. This also assumes no quantities have yet been delivered into stock (SOSTOCK->QTYD) because those quantities get extracted from what's outstanding to receive.