Only front page works
I believe the problem was due to the location of my .htaccess file. (The main problem was the Clean URLs.)

My original location was /var/www/drupal/sites/default/files; after, I moved it to 
/var/www/drupal. The .htaccess file started to being read. Previously, it was not being read.

I hope this helps someone else! This also may not be the solution because I am having a different issue now.