Have you added the BBC Sounds service to Sonos AND the Sonos skill for Alexa to Alexa AND the BBC Sounds skill for Alexa to Alexa ?
EDIT: The fact that you talk about ‘enabling and disabling’ the BBC Sounds account makes me wonder if you are mistnterpreting an instruction to do something in the Sonos app, and have instead been enabling and disabling skills in the Alexa app.
So, in the Sonos app, have you linked your BBC Sounds account to your Sonos system?
Without seeing the exact message and where it originates there is a bit of guesswork on my part here.