Fatal error on Drush run
I suggest you to run -

composer self-update
composer update


It might solve your problem, but if it doesn't then try to install latest version of drush. I installed drush using https://www.drupal.org/node/1248790.
However, there is a full documentation on how to install drush for specific operating system. Link to documentation - https://www.drupal.org/node/1791676