I tried what you have suggested;
* PO was created in the long process (i.e. PR >> Autocreate RFQ >> Copy RFQ to Quotation >> Copy Quotation to PO)
* "Navigation: Requisitions -> Requisition Summary >> Go to "Related Documents" tab and enter the Purchase Order number."
The purchase order number is not accepted. It says 'Invalid value for field REATED_PO
but when i search the same PO number from PO summary it opens.