I have traditionally used ansible-galaxy install williamyeh.fluentd to install this role... and that still works. But if you go to the Galaxy role page for this role (https://galaxy.ansible.com/william-yeh/fluentd), it shows:
ansible-galaxy install william-yeh.fluentd
...which does not work. You have to still use williamyeh for the namespace. This may be an upstream issue with Galaxy.