I had a very similar problem for months and literally just resolved it, so I thought I would share what worked for me.
In my case, I had Google Assistant enabled in the past but it somehow suddenly disconnected and kept giving the same error you got when I attempted to reconnect it. I went through the process of enabling Alexa (which forced Google Assistant to removed from the Sonos system because Google and Alexa can’t both be enabled at the same time), and once it was removed from the system, I was able to disable Alexa and re-enable Google. Maybe this will also work for you?
Good luck!
Isn’t the One SL the version without a microphone? How can you add any voice assistant? Although you would think Sonos would offer a more specific warning in that case.