Once configured and restarting successfully on Apache. just it is not sure where you need serve from, so it's choosing your localhost ​( by default. To avoid this message, that you want to serve from localhost (, just the below line in Apache configure file.

* Restarting web server apache2                                                                                                                      
 AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using Set the 'ServerName' directive globally to suppress this message  [ OK ]

Open the configure file and appand a line,

#  vim /etc/apache2/apache2.conf

ServerName localhost

Finally, restart the Apache service.

# service apache2 restart