This error message appears when you try to use a column in the SELECT list that does not appear in an aggregate function or in the GROUP BY clause.
Errors of the Severity Level 16 are generated by the user and are corrigible by the user. The statement cannot be executed this way. You must either use this column in an aggregate function or add it to the GROUP BY clause.
So you can try something like this;
select ItemCode,ItemDescription,sum(S36) as ,sum(S38) as ,sum(S39) as  from SalesStockRM where DayDate>='09/11/2010' and DayDate<='11/11/2010' and Pieces='Shirt' GroupBy ItemCode,ItemDescription
However, you need to verify on the logical level if they return the expected resultset.