>>Try
>>

>>SELECT SI.AccountCode, MAX(SI.InvoiceDate) AS Latest  
>>	FROM SalesInvoiceItem IV 
>>		INNER JOIN SalesInvoice SI ON IV.InvoiceNumber = SI.InvoiceNumber
>>	WHERE ProductCode='56.GTR003.001.01' AND MAX(SI.InvoiceDate) < DATEADD(MONTH, - 1,GETDATE())
>>GROUP BY SI.AccountCode
>>

>>
>
>Sergey, you can't use MAX in a WHERE clause. Only in a HAVING, or within context of a subquery. (And I see Naomi just posted the same thing)
>
> Do you test code before posting it? :)

So replace WHERE with HAVING. What is a big deal?