This might be because you are sending a mutation for updating a purchase order.
This mutation has a required argument that is po_id, because it needs it to identify the purchase order you want to update, but you’ll also have to insert another field (the field you want to update), as po_date for example.
On the other hand, if you want just to look at info about that specific purchase order, you’ll have to use a query for it, instead of a mutation (purchase_order query for example)