Drush not recognized and internal or external command
Did you install Drush Launcher? This seems like a PATH issue -- your command lines doesn't know where to find Drush 9.


  To be able to call drush from anywhere, install the Drush Launcher.
  That is a small program which listens on your $PATH and hands control
  to a site-local Drush that is in the /vendor directory of your
  Composer project. If you skip this step, run Drush from Drupal root
  via ../vendor/bin/drush. In that case Drush's bash integration and
  custom prompt won't work.


from: http://docs.drush.org/en/master/install/