If Anonymous authentication is not enabled for your application, you will encounter the error:
authentication via 'anon-user' is unsupported.
To correct this issue, enable Anonymous authentication using the Realm UI (or Realm Admin API).
1. In left-hand navigation of the Realm UI, click Users -> Providers.
2. Click EDIT to update the Allow users to log in anonymously provider.
3. Toggle Provider Enabled to ON and click Save.
4. Click Review & Deploy in the top blue banner to deploy the change to your production application. For more information on reviewing and deploying changes, see Deploy from the Realm UI and Why are the user modifications made to the Realm UI not reflected in the Realm Application?.
5. After the changes are deployed, the Allow users to log in anonymously provider displays as enabled.