Thanks Prasanth for your reply.
Yes, i checked the sales order change log. It was initially credit block after that it was released. then PR-PO-MIGO created. Post PO, no changes were done to sales order.
BTW, even though TAS billing relevance is B( order related ) because of copy control VTFA the billing qty is E which actually copies the Goods receipt qty and it gives an error as below while creating billing with OUT MIGO is done.
"No open billing quantity has been determined"
thanks...