drush @sites command doesn't work
"@sites" is no longer available with Drush 9. I presume this what is being used with Drupal 8 here.

More information here:
https://github.com/drush-ops/drush/issues/3019#issuecomment-334503217


  Drush 9 no longer supports executing a single command on multiple sites with alias lists (@A,@b,@c) or @sites. Neither of those syntaxes are available in any context. You may still use drush sa @group to see all aliases in a group; that is the only context in which Drush recognizes alias lists.