Hello Shajan,
I don't know, what you mean by "payment order number", I couldn't find a field of that name. Did you mean "purchase order" by any chance...? If so, the field is empty in the corresponding vendor line item as the purchase order number & item is only populated in the corresponding GR/IR account item. If you want it in the vendor items, you will have to populate it yourself by implementing custom code using either BadI FAGL_ITEMS_CH_DATA or Business Transaction Event 00001650 (P&S Interface).
Please be aware, that the vendor item can span several purchase orders, if one invoice covers items from different POs.
Weiterhin viel Freude mit SAP...
Cheers
MrB.