NameVirtualHost has no effect

What do you think would be solution to my problem?

user@admin:~$ sudo service apache2 restart
sudo: unable to resolve host admin
* Restarting web server apache2                                                
AH00548: NameVirtualHost has no effect and will be removed in the next release
/etc/apache2/ports.conf:8

From the apache documentation:

Prior to 2.3.11, NameVirtualHost was required to instruct the server that a particular IP address and port combination was usable as a name-based virtual host. In 2.3.11 and later, any time an IP address and port combination is used in multiple virtual hosts, name-based virtual hosting is automatically enabled for that address.

This directive currently has no effect.

The solution to your problem is to remove the NameVirtualHost directive.