>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
>

You can not use MAX() function in the WHERE clause, only in the HAVING clause.