On my newly installed Drupal 8.4.4 sandbox installation, I've installed Drush 9 and the Devel module:
composer require drush/drush:^9 composer require drupal/devel
After running these commands, I installed the Devel module using the Extend tab in Drupal.
Drush seems be be installed correctly:
www-data@39e134dc7690:~/html$ /var/www/html/vendor/bin/drush version Drush version : 9.1.0
The Devel modules seems to be installed correctly:
www-data@39e134dc7690:~/html$ grep -r devel-container-service /var/www/html/modules/contrib/devel/ /var/www/html/modules/contrib/devel/src/Commands/DevelCommands.php: * @aliases devel-container-services,dcs,devel-services /var/www/html/modules/contrib/devel/drush/devel.drush8.inc: $items['devel-container-services'] = array(
But Drush doesn't seem to find the devel module:
www-data@39e134dc7690:~/html$ /var/www/html/vendor/bin/drush devel-container-services [Symfony\Component\Console\Exception\CommandNotFoundException] Command "devel-container-services" is not defined.
Are there any path settings I need to tweak to get Drush to find the Devel module?