Please How to fix "Invalid use of group function" - mysql

Question

I try to use nested CASE WHEN syntax in SUM() function but nothings works since 3 days
When I retrieve all sum and nested conditions the code work great
I explain:
Table contrat: Each client have a contract in which this two values: Prix_TTC (the price of the article), Qac_cont (The quantity of the article client must consume each month)
Table commande: Each client order article with quantity (Quantite_cmd)
So what I try to do is to get all financial loss for each client during the actual month and SUM all these amount
SELECT
SUM(pertes) AS pertes_reelles
FROM
(SELECT
c.ID_clt,
SUM(CASE
WHEN
((co.Qac_cont - SUM(CASE
WHEN Quantite_cmd IS NULL THEN 0
ELSE Quantite_cmd
END)) * - 1) < 0
THEN
((co.Qac_cont - SUM(CASE
WHEN Quantite_cmd IS NULL THEN 0
ELSE Quantite_cmd
END)) * - 1)
ELSE 0
END) * co.Prix_TTC AS pertes
FROM
clients c
LEFT JOIN contrat co ON co.clt_ID = c.ID_clt
LEFT JOIN commande cmd ON cmd.clt_ID = c.ID_clt
AND MONTH(DATE(cmd.date_livr_cmd)) = MONTH(DATE('2019-08-30 09:23:23'))
WHERE
c.Etat_clt <> 'D'
GROUP BY c.ID_clt) AS liste

Answer 1

Like #BobJarvis says, the problem was with the nested SUM so I have broken down nested processing and it WORKS Thanks ! the new request :
SELECT SUM(CASE
WHEN
perte < 0
THEN
perte * - 1
ELSE 0
END) AS pertes_reelles
FROM
(SELECT
c.ID_clt, co.Prix_TTC, ((co.Qac_cont - SUM(CASE
WHEN Quantite_cmd IS NULL THEN 0
ELSE Quantite_cmd
END)) * - 1) as ecart, co.Prix_TTC * ((co.Qac_cont - SUM(CASE
WHEN Quantite_cmd IS NULL THEN 0
ELSE Quantite_cmd
END)) * - 1) as perte
FROM
clients c
LEFT JOIN contrat co ON co.clt_ID = c.ID_clt
LEFT JOIN commande cmd ON cmd.clt_ID = c.ID_clt
AND MONTH(DATE(cmd.date_livr_cmd)) = MONTH(DATE('".$today."'))
WHERE
c.Etat_clt <> 'D'
GROUP BY c.ID_clt) AS liste