Is there a way to group the output of a query based on two rows I'm not sure my question is asking the right question, so i will be as specific as possible in here.

I have the following data which i have changed the values of.


Below is the code I am using to pull this data through.

/*this is used for the open order book*/ 
SELECT co.[slsman]               AS Salesman, 
       co.[cust_num]             AS "Customer Number", 
       ca.NAME                   AS "Customer name", 
       co.[co_num]               AS "Customer Order", 
       ci.item, 
       ca.curr_code, 
       ci.price * ci.qty_ordered AS value 
FROM   [co] co 
       LEFT JOIN custaddr ca 
              ON co.cust_num = ca.cust_num 
                 AND co.cust_seq = ca.cust_seq 
       LEFT JOIN coitem ci 
              ON co.co_num = ci.co_num 
WHERE  co.co_num LIKE 's%' 
       AND co.stat = 'o' 
       AND co.cust_seq = '1' 
       AND item NOT LIKE '%carriage%' 
       AND ci.stat NOT LIKE 'f' 
       AND ci.stat NOT LIKE 'c' 
ORDER  BY co.slsman, 
          co.cust_num 


What I am trying to do is merge all rows where the Customer order and the item part number are the same, and sum all the costs of these rows. I have tried adding a group by before the order by, but i understand after some reading that this does not work how i thought it did. 

GROUP BY  co.co_num,
          ci.item


which gives the following error


  Msg 8120, Level 16, State 1, Line 3 Column 'co.slsman' is invalid in the select list because it is not contained in


either an aggregate function or the GROUP BY clause.

Below is what i want to be outputting. As you can see, i don't want to merge any cell text values, just the value one where customer order and item are the same.
SELECT MAX(co.[slsman])               AS Salesman, 
       MAX(co.[cust_num])             AS "Customer Number", 
       MAX(ca.NAME)                   AS "Customer name", 
       co.[co_num]                    AS "Customer Order", 
       ci.item                        AS item, 
       MAX(ca.curr_code)              AS curr_code, 
       SUM(ci.price * ci.qty_ordered) AS sum_value 
FROM   [co] co 
       LEFT JOIN custaddr ca 
              ON co.cust_num = ca.cust_num 
                 AND co.cust_seq = ca.cust_seq 
       LEFT JOIN coitem ci 
              ON co.co_num = ci.co_num 
WHERE  co.co_num LIKE 's%' 
       AND co.stat = 'o' 
       AND co.cust_seq = '1' 
       AND item NOT LIKE '%carriage%' 
       AND ci.stat NOT LIKE 'f' 
       AND ci.stat NOT LIKE 'c' 
GROUP BY  co.co_num,
          ci.item
ORDER  BY Salesman, 
          "Customer Number"