Cron job giving error while using usermgr utility

I have to generate list of users present in system daily. To automate this process i have scheduled a cron job that runs this script to extract userlist daily.

So this is my script a.sh and under this i have added :

usermgr -d $GLOBAL_DOMAIN -list > file.csv

In my crontab, i have added following syntax :

30 06 * * * /bin/sh /home/user/a.sh

Now when this cron jub run, it returns an error stating usermgr : command not found.

Can you help me understand why it's giving such error while executing command via cron when the same command works fine without cron.