Use a query's value in where statement
The answer is to put any testing of aggregations in to a HAVING clause:

SELECT
    S_Product.Name AS "Name"
    SUM(S_Item.Quantity) AS "QtySold"
FROM s_product
INNER JOIN S_Item ON S_Product.ID = S_Item.Product_ID
GROUP BY S_Product.Name
HAVING SUM(S_Item.Quantity) &gt; 500 ;