I have been following this thread because I am in the same situation with having an Echo and having room(s) that have multiple devices/switches that for years I have been controlling (turning off/on/dim) by using a Scene in my ISY -- eg. my family room / sun room has about 6 switchlinc switches and a few lamps that make up one Scene. After I purchased the Echo and set up the ISY Portal, I just mapped that Scene to "Family Room" and it turns on and off the entire Scene (family room) no problem -- but of course no dimming was possible, as this thread has been discussing ("that command doesn't work on device 'family room'").
So, as others have mentioned, I put all of the devices that were in my Scene into a "Group" in the Alexa app and sure enough you can say "Set Family Room to X pct" and it works great -- sets the dim level appropriately. That part is GREAT. What happens though, is that it sends the commands to EACH DEVICE INDIVIDUALLY (of course it does!), whether you are turning the lights on, off, or dimming them. So at least you CAN dim them, but now when turning on/off the family room lights, it takes longer, and it looks strange, doing them one at a time. So now I need 2 different "names" for the Family Room (one for the Scene and one for the Group) if I want to continue to turn all of the lights on or off ALL AT THE SAME TIME, and still be able to dim them to any brightness level.
Just thought I would throw that out there since I have not seen anyone make mention of this... or maybe there is something I can do different/better.
Stuart