This error happens because your server doesn’t have ServerName Apache 2 variable defined and reverse lookup on your IP address returns nothing. We can specify ServerName as 127.0.0.1 to make this message disappear. Just open your favorite terminal emulator and paste:
echo "ServerName 127.0.0.1" | sudo tee -a /etc/apache2/conf.d/server_name
Restart Apache
sudo service apache2 restart
