I never used the dashboard (see [1]) for fwaas but I would check first the devstack plugin and the readme for that:

As I see perhaps the only change you need is to change enable_service q-fwaas-v2 to enable_service neutron-fwaas-v2

Just a note the dashboard code is just minimally maintained so I am not sure if in its current state it can work with fresh Horizon

(like they in the process of refreshing django version etc...)

[1]: https://opendev.org/openstack/neutron-fwaas-dashboard

Best wishes

Lajos (lajoskatona)