Informix follows the strict SQL standard and requires all non-aggregate values in the select list to be listed in the GROUP BY clause. Thus, the SQL must look like:
SELECT MONTH(startdatetime) AS Month, COUNT(sessionid) AS TotalCallsOffered,
CASE
WHEN MONTH(startdatetime) = 5
THEN 'SUSUKUDA'
END AS Susukuda
FROM agentconnectiondetail
WHERE MONTH(startdatetime) = 5
GROUP BY Month, Susukuda
ORDER BY Month ASC
I've chosen to name all three columns with AS and to GROUP BY those names (and ORDER BY them too).