Set the documents to Auto-Approve.
Purchasing SuperUser --> Setup --> Purchasing --> Document Types --> Purchase Order (do same for Requisition) --> Check Owner can Approve.
Initially PR preparer or PO Buyer does not have the Document approval rights.
He can only approve the document (PR/PO) , if you assign the approval rights otherwise he can only forward the document to next level in position hierarchy.
Thank you, i did that and it worked.