Best answer by Ryan S
Make sure that you've signed out of all Amazon apps on your mobile device as this error can come up if you're using two Amazon accounts and the wrong one gets cached.
If you're still having trouble, there are some really great Alexa and Sonos troubleshooting steps to start with here.
Finally, I'd encourage you to give us a call on our support line if you're still not in a good place. We're a bit busy, but we're here to help and will get to everyone as soon as possible.