See what path aliases are set up with Drush?
You have to either be within a drupal doc_root (aka apache verbiage) or have properly configured drush site aliases for commands to work:

Eg, if your site lives at /var/www/

cd /var/www/
drush cc all


Or if you have setup a drush site alias of @local, then from anywhere on your system:

drush @local cc all