Hi,
Cross check did valuation area is defind as Plant in t.code:OX14 !
Now active negative stock in t.code: OMJ1 and also active negative posting permitted for company code in t.code:OBY6.
Refer details in a thread which alreday discussed for Negative stock posting:
http://scn.sap.com/thread/3194878
Regards,
Biju K