Sorry, but Nagios is currently not checking for external commands, so your command will not be committed!

This message comes on nagios dashboard when externals commands are disable.
To enable external commands.Edit the nagios.cfg file and enable the value of check_external_commands parameter.
Change the value from 0 to 1

In Debian or Ubuntu:

vi /etc/nagios3/nagios.cfg

check_external_commands=1

OR

In CentOS or Redhat

vi /etc/nagios/nagios.cfg

check_external_commands=1

Restart the nagios

Restart the Nagios in Debian or Ubuntu

/etc/init.d/nagios3 restart

Restart the Nagios in CentOS or Red Hat

/etc/init.d/nagios restart

Now issue must be resolved.
(Except there is a bug in Ubuntu 12.04 LTS,it has the solution also.I am writing post for it )