Clean URLs with Apache 2 on Ubuntu

Open terminal: Ctrl+Alt+T

1)  sudo a2enmod rewrite
2) sudo apache2ctl -M   (and seeing if rewrite_module is on the list.)
3) sudo gedit /etc/apache2/sites-available/default
4) change text "AllowOverride None" to "AllowOverride All"
5) sudo /etc/init.d/apache2 reload

If you are running Ubuntu 14.04+:
sudo gedit /etc/apache2/apache2.conf
Otherwise:
sudo gedit /etc/apache2/sites-available/default

Refer: https://www.drupal.org/node/134439