Drush not working properly on Ubuntu
Sounds like you are not using the right version of Drush.  For Drupal 8.0-beta7, you need Drush 7.0.0-alpha9.  For Drupal 8.0-beta6, you need Drush 7.0.0-alpha8.

Drupal 8 APIs are changing rapidly; if you have mixed versions, you can easily encounter spurious problems.  Tracking HEAD of Drupal 8.0.x and Drush master is another viable strategy.