How does module_enable works in Drush?
module_enable is a Drupal 7 function - it would seem you're not running this command on a drupal 7 site.

Downgrading Drush won't have any effect, if you want to use this extension with Drupal 8 the code will need to be updated to satisfy the new API.

The new equivalent is:

\Drupal::moduleHandler()-&gt;enable(['civicrm']);