Dear Friend
I think
SELECT distinct COUNT(S.INVOICENO) AS LESSTWO , YEAR(S.invoicedate)As YER,Month(S.invoicedate)as MNTH
FROM SALESDATA S where S.BILLINGTYPE='INVOICE' AND (S.invoicedate between '4-1-2009' and '4-30-2010')
AND S.TAXABLEAMT <=2000
GROUP BY YEAR(S.invoicedate) ,Month(S.invoicedate) ORDER BY YEAR(S.invoicedate) ,Month(S.invoicedate)
this will solve the issue
If there is still causing the error then feel free to contact along with the table exact structure and the error you got along with the output needed
regards
Vipin Kumar Mallaya G