Drupal 8 - Drush composerRequireDrupal8 error
Please do not install Drush using composer global require.


Quoted from http://docs.drush.org/en/8.x/install-alternative/.

If you have Drupal 8 installed conventionally best practice would be to install Drush 8 globally by downloading the latest drush.phar, see http://docs.drush.org/en/8.x/install/.

If your Drupal 8 site is composer based then Drush 9 is probably already included. If not see http://docs.drush.org/en/master/install/.