I’m dealing with exact same issue from a certified refurbished model. I bought another one new and did not run into any of these issues. Perhaps it has to do with the recently updated app? 
 

im totally stuck at the moment and seeing this “beam not registered”

And this was fun - i tried calling support and was quoted a 110 minute wait time. 

I just purchased sub and amp. Having the same issuing registering on my existing system with several other devices working. 

It’s the app. Tried on the updated app and didn’t work. Tried on my other phone with the non updated version and registered no problem at all

well it's a shame all of my devices updated automatically ...

I confirm, using an older app version fixed the problem. The app complained about being outdated but eventually by clicking erratically through options, the register product option became ungreyed out and worked within seconds.

I don’t have an older app version. I have the same problem, can you download an older app version?

Just got myself a new Beam gen 2 today and just now I finally got it to register. 
 

Tried everything and this how it worked for me. Hopefully this works for others.

Inside the app under your account go to App Preferences, scroll down to reset app once you do that it’s going to show the login screen. Once you sign in again it will show join existing group and when in my beam was registered. 
 

It’s the app, but there’s a workaround that worked for me…

https://en.community.sonos.com/speakers%2D229128/found%2Da%2Dworkaround%2Dfor%2Dthe%2Dproblem%2Dcaused%2Dby%2Dnew%2Dsonos%2Dcontroller%2Dapp%2Dwhen%2Dadding%2Da%2Dnew%2Dspeaker%2D6892626

Transfer your current system to yourself works!!!  Great hack!