Nagios External Command Interface

check nagios.cfg 
check_external_commands=1

in nagios configure directory cgi.cfg 

authorized_for_all_services=nagiosadmin
authorized_for_all_hosts=nagiosadmin

And

authorized_for_all_host_commands=nagiosadmin
authorized_for_all_service_commands=nagiosadmin

restart nagios and aphache as well.

check /etc/group

nagios group should be like

nagios:x:1001:www-data

set permission for var/rw/*.* in nagios configure directory to make www-data useable.

chmod g+x /var/rw

everything should be working with no longer error.