Are you able to log on to your my.avast.com account from your browser ?
If so it does seem to be pointing to an issue with the beta - I'm not on this beta, but I think this could also be an issue with my.avast.com. When I log on using my browser, this PC isn't displayed as a device.
After a couple of attempts I was able to connect to my.avast.com account from the program user interface. Even this device isn't shown.