This Serial Number is Not Available error while receiving an PO

Hi ,

I am doing ST-OT transaction . SThas already Sales updated and OT need to receipt.

So while performing the receipt transaction , i am getting "This Serial Number is Not Available error" although for respective item + BP setup shows Serial No. Required is "N".

Still i am getting this issue while receipt the PO .

below are the flag are present in Item and item + BP setup

Serial No. Required    Serial Number Not Required

Lot Status Code   Not on Hold

Lot Process Type   Serial No Assigned Manually