There are a few possible reasons for the issue you're facing, as further detailed below ...
Drush commands need to be issued from within the correct subfolder
A "possible" reason might be because Drush commands must be issued from the appropriate subdirectory of your website (change to the directory where your Drupal site is installed and try again).
So try navigating to the sites/default folder (assuming this is NOT a multi-site), and then retry your Drush comand.
Use the Drush version that supports your Drupal version
It could well be an issue with the version of Drush that you are using. Check if it is an option to downgrade to a lower version of Drush? FYI: Drush 7.x does support D8 also ...
Note: in one of the comments in the question, the OPer wrote "Drush version is 4.5". Haven't double checked it yet, but that appears to be an extremely old version of Drush. I wouldn't be surprised if that version doesn't support D8 "yet". And since the D8 folder structure changed quite a bit, that would also explain the error like "A Drupal installation directory could not be found" ... More research (and clarifications from OPer) are seems appropriate here ... (I might update my answer here later on with some relevant pointers about "which version of Drush supports which version of Drupal".