Hi,
Negative posting used for a certaion special purpose, you need to have following settings:
Select check box of "negative stock perimitted" in processing parameters segment in OBY6 t.code for your company code.
AND
Also select check box of "negative stock" for your plant(valuation area) in OMJ1 t.code.
But why you want go for Negative posting process?
Regards,
Biju K