Please let me know if my version is lesser than the one that has
listagg.

$> db2 "with temp1(a) as (values('a'),('b'),('c')) select listagg(a)
from temp1"
SQL0440N No authorized routine named "LISTAGG" of type "FUNCTION"
having
compatible arguments was found. SQLSTATE=42884
$>db2level
DB21085I Instance "bculinp" uses "64" bits and DB2 code release
"SQL09071"
with level identifier "08020107".
Informational tokens are "DB2 v9.7.0.1", "special_25512",
"IP23034_25512", and
Fix Pack "1".
Product is installed at "/opt/IBM/dwe/db2/V9.7".

LISTAGG is supported on DB2 9.7 FP4 for LUW.