Unable to install modules with Drush I've got a fresh install of the Drupal 8 Beta 12. I'm trying to install the REST UI module:

$ drush dl restui-8.x-1.x-dev

PHP Fatal error:  Class 'Drupal\Core\Session\AccountInterface' not found in /var/www/html/drup/core/includes/bootstrap.inc on line 64
Drush command terminated abnormally due to an unrecoverable error.   [error]
Error: Class 'Drupal\Core\Session\AccountInterface' not found in
/var/www/html/drup/core/includes/bootstrap.inc, line 64


Does anyone know what's going on here?
Drush 6 does not support Drupal 8 and you need to upgrade to drush 7 (well I just upgraded to drush 8):

Drush Version   :  8.0-dev


See duplicate issue: Running Drush returns an error

Edit: If you are working on Drupal 8 you need the Drupal Console its syntax is a bit long-winded, but it is incredibly powerful.