GR non-valuated flag has been set to 'X'

What causes this issue?

You entered the special stock indicator on the SPEC2000/LB tab page,
which means that you are using the SAP for Aerospace & Defense
subcontracting solution with special stock.

System Response

The special stock on subcontractor side is non-valuated stock (except
for stock type E - sales order stock, Q project stock, O - plant stock
with vendor), therefore the goods receipt for this purchase order item
should also be non-valuated. The system automatically set the
corresponding indicator (GR non-valuated) on the PO item (on the
Delivery tab page).

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.