Hi ranjeet...
if you want to see the list of PO's for the particular Vendor ..
1. Use ME2L - Tocde : Enter Vendor and excute....it will populate list of PO's for that Vendor.....select the specific one and do the invoice processes...
2. Use s_al0_96000493 - Tcode .......Enter vendor number and execute ---to get detailed view
As per your query - IPO is invalid means ::::Explain you scenario for further discussion....