To save us guessing, what have you tried - starting with: have you added Alexa yet? 

Hi

yes i have added Alexa, created a group called family room containing my ultra and sub 4. I have added the ultra to my Alexa.

when I ask it to play something on Family room or Arc Ultra, I get told to enable the skill in Alexa and ask her to discover it.

Where do you go wrong in these steps.

https://support.sonos.com/en/article/set-up-an-amazon-alexa-device-to-control-sonos

A Sub should be bonded to the Arc Ultra, and will then appear as a room in the Sonos app. If you need to “create a group called family room” something is wrong in your setup. 

I had the same issue yesterday, for me and I hope for you also the solution was only disable the skill of Sonos and after enabled it again and connect your sonos account it alone will connect all your speakers and enable Alexa, but before this step you need to link your account of Alexa in the Sonos app :)