Drush 9 does not work with drupal 7, but drush 8 does work with it. I installed it by creating a new ubuntu user and making sure composer is installed globally then install cgr via:
composer global require consolidation/cgr
then add to your ~/.bashrc file
cgr drush/drush:8.x-dev --prefer-source
Then I followed the tutorial here: https://docs.drush.org/en/8.x/install-alternative/
Install a global Drush via Composer
Follow the instructions below:
Install Composer globally.
Install the cgr tool following the instructions in that project.
Add composer's bin directory to the system path by placing export
PATH="$HOME/.composer/vendor/bin:$PATH" into your ~/.bash_profile (Mac
OS users) or into your ~/.bashrc (Linux users).
Install latest stable Drush: cgr drush/drush.
Verify that Drush works: drush status
Please do not install Drush using composer global require. See Fixing
the Composer Global command for more information.