Drush ard is working for D6, not for D7 > bootstrap error
This seems like a permissions' issue on your server. Make sure that the user that you are running the command as, has the appropriate permissions in the Drupal 7 directory.

An easy way to fix it is change the owner of the Drupal7 dir by using something like:

chown -R username:usergroup /path/to/drupal7/dir


or if you are working in a debian based environment you can run the command with super-user-do like:

sudo drush ard