Errors: apache2: Could not reliably determine the server's fully qualified domain name, using x.x.x.x for ServerName

Solution:

To fix that problem, you need to edit the httpd.conf file. Open the terminal and type,  

sudo gedit /etc/apache2/httpd.conf

By default httpd.conf file will be blank. Now, simply add the following line to the file.  
ServerName localhost

Save the file and exit from gEdit. Finally restart the server.  
sudo /etc/init.d/apache2 restart