CST is not being calculated on PO

Dear All,

I have a problem in totals of purchase order. Even after defining all proper setups, CST is not calculating in purchase order. The Any suggestion.

I have checked the form type, states, address, item tax group, tax group etc.

If I try to post the PO "Field Registration number must be filled in.’ message is coming.

I am able to post the PO only without CST.

Thanks in advance.

I WANT TO ADD MORE IN ABOVE QUESTION.

CST IS BEING CALCULATED CORRECTLY IN SALES ORDER.

PLEASE SUGGEST ON ABOVE.