Hi
I found a silly way to solve the problem but it's definitely a bug and should be solved with its developers.
lets start :
1. Install the iscsitarget.
2. Enable and apply configuration changes. (in this step you will see the "Failed to execute command 'export LANG=C; monit monitor iscsitarget 2>&1': monit: action failed -- There is no service by that name" error.)
3. Restart iscsitarget service in CLI (/etc/init.d/iscsitarget restart)
4. Update your OpenMediaVault from Update Manager page.
5. Now you can start iscsitarget service without error.