Assuming Spotify is available in your Country on Sonos and that your online Sonos/Spotify country settings are set correctly for your location, then perhaps try the following.. go online and login to your Spotify account via a web browser and look for and sign-out of all devices on your account. Then remove/reinstall the Spotify App from your mobile controller device and finally do the same thing and remove reinstall the Spotify service in your Sonos App. Just ensure that when logging into your Spotify ‘account’/‘service’/‘app’ that you use the same login credentials.
Then see if that solves your issue. If not, it maybe best to contact Spotify support, or perhaps Sonos Customer Care via this LINK