conditions

In sales order I have given manually condition type for VAT

it is getting calculated and showing value 150000

but when i go for billing and check the conditions there...i see JVAT missing there and giving me value as 144000

so when I tried to enter manually, it is saying

"The condition cannot be entered at header level because it is not marked in Customizing as being a header condition."

what has to be done? Any steps?