CiviDiscount compatibility with CiviCRM 5.9
Yes, CiviDiscount is compatible with CiviCRM 5.9.

I have a 5.9 environment with Drupal and is working as expected.

Your issue could possibly be permission related.
CiviDiscount recently introduced a permission - administer CiviDiscount

Joomla permission documentation -

https://docs.civicrm.org/user/en/latest/initial-set-up/permissions-and-access-control/#permissions-in-joomla

Would also advise to clear CiviCRM caches –

/administrator/?option=com_civicrm&amp;task=civicrm/clearcache

/administrator/?option=com_civicrm&amp;task=civicrm/admin/setting/updateConfigBackend&amp;reset=1