GROUP BY in SQL query is giving syntax error when used with Order By

I have this query:

SELECT customer.cus_id,customer.fname,customer.lname,customer.image, message.message ,message.msg_id,message.from_cus,message.to_cus, message.sent_time
FROM customer 
INNER JOIN message ON cus_id=message.from_cus OR cus_id=message.to_cus
WHERE customer.cus_id IN ( 31,26)
AND   (
from_cus =25
OR to_cus = 25
)
AND customer.cus_id NOT IN(25)
ORDER BY    message.sent_time DESC
GROUP BY customer.cus_id ;

where 31, 26 are the cus_id of the customers whose last messages I want to fetch.I am getting all the messages sorted asscording to time between the 1 and ( 31,26 ) customer if I remove GROUP BY constraint. But adding it gives me this syntax error

[Error code:1064 SQL state:42000] You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'GROUP BY customer.cus_id' at line 11

I have disabled ONLY_FULL_GROUP_BY SQL mode. Still I dont know How to fetch the last message.

Answer

This is not an aggregation operations, this is a filtering operations.

Use window functions:

SELECT cm.* FROM (
        SELECT customer.cus_id,customer.fname,customer.lname,customer.image, message.message ,message.msg_id,message.from_cus,message.to_cus, message.sent_time,
        ROW_NUMBER() OVER (PARTITION BY customer.cus_id ORDER BY message.sent_time DESC) as seqnum
        FROM customer 
        INNER JOIN message ON cus_id=message.from_cus OR cus_id=message.to_cus
        WHERE customer.cus_id IN ( 31,26)
        AND   (
        from_cus =25
        OR to_cus = 25
        )
        AND customer.cus_id NOT IN(25)
    ) cm 
WHERE seqnum = 1
ORDER BY msg_id DESC;