You got that warning because the user you are logged in does not have write
permission on the folder /bitnami/drupal/sites/default/files/css/
. So, drush (running as that user) cannot delete the aggregated css files while rebuilding the cache.
To solve this, you need to set the permissions of that folder to 775 and make sure that the user you are logged in with belongs to the group-owner
of it.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…