Modification of Inbound consignment (Purchase Order) not allowed after submission is to prevent the discrepancies between created consignment and actual delivery because supplier maybe gets the updated consignment in time and deliver as per the amended consignment.
This is also to prevent integration impact with ERP system where created Inbound consignment (Purchase Order) cannot be amended. The Inbound consignment (Purchase Order) has to be canceled and re-raised with the PO number.
We hope the details above were helpful for you. Please feel free to share with us if require further assistance and will be happy to take it up.