Let me know that Item code is of single or multi segments ?
check the following in item master.
(1) Item is active or not.
(2) Item is assigned to your inv.org. or not.
(3) Item is purchasable or not.
(4) Trim the spaces in item code if any
i did all of these, still not working
Can you just share the screen shot of Item Master with Item details and screen shot of Requisition with an error message ?