In NACE tcode you select Appliction EF-purchase order click condition records(F8) &doubl click your output type and Here select Key combination document type based or purchasing organization based. document type or pur.org partner function (DP-delivery plant)
Help full U.
Select the "check box" to your PO output(message) type which used to take print output automatically OR getting to be be triggered automatically on PO creation in the following path for Indicator: Print Operation (1) & save.
SPRO -> MM -> Purchasing -> Message -> Output control -> Message types -> Define message type for Purchase Order -> Fine-Tuned Control: Purchase Order
Also make sure that you must have Partner Function:VN(Vendor) in "Default values"TAB of you PO ouput type in following path:
SPRO -> MM -> Purchasing -> Message -> Output control -> Message types -> Define message type for Purchase Order -> Maintain Message Types for PO
Keep condition record for PO output type with Partner Function:VN(Vendor) in t.code:MN04.