When working in Drupal 8, after installing Composer, and then you type "composer require drush/drush" in hope to install drush at the root of verdor/bin folder. Surprisingly after Composer shows Drush 9 has been installed and you try to type "drush status" and the error comes up that "drush commands is not known"; Make sure you uninstall Composer that might be installed on a global level with this command: sudo apt-get remove composer. Make sure when running this command you are at the root level of Ubuntu Operating System by first running "cd /" After Composer has been removed, navigate to Drupal vendor/bin and run these commands: "sudo apt-get install composer", after Composer has been installed, run this command "composer require drush/drush" and Drush will be installed in the root level.