Hi Anbuvanan -
Can you check - BADI - ME_PROCESS_PO_CUST, Method - PROCESS_ITEM,
This will trigger in case any changes is done in the line item. You can check the fields if blank, you can raise errors.
Also, BADI - ME_PROCESS_PO_CUST, Method - CHECK
It will trigger when SAVE or Check button is clicked . You can check the fields if blank, you can raise errors but this is check for PO document as a whole.
Let us know, in case any further help.
Regards,
Atul Mohanty