Drush not recognized and internal or external command When I update Drush to Drush 9.1.0, the following error now appears:

'drush' is not recognized as an internal or external command, operable program or batch file.


This issue has occurred on three different machines: two running Windows Server 2016 and one running Windows 10. I've read that it could be due to mismatched global and site Drush versions, but both versions are at 9.1.0. When I downgrade to Drush 8.x.x, it functions as normal. Both the global and site-specific Drush installs were installed with Composer. Are there any fixes for this error?
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/