This is a frequently asked question. Your script has to dot in the db2profile for the correct db2-instance because cron won't do that. Remember that you should never run normal db2 commands as root (except for install/update/upgrade/licence/instance-create/drop etc), instead run the script as an appropriate user.
In your reply you are saying "Your script has to dot in the db2profile for the correct db2-instance" means how exactly i have to call db2profile? On some environment i referred then i tried /home/instancename/.profile for this got error that no such file or directory. Can you please clarify how to create db2profile?