Has the tenant you are using the app in consented to the application? If not please follow https://learn.microsoft.com/en-us/azure/active-directory/manage-apps/grant-admin-consent
The beta warning basically says that changes can be made to API. We tend to avoid any breaking changes but they are certainly a possibility in /beta (vs /v1.0 where we do not make such changes, but that also means you might not get new features).
Apps in themselves are not production or non-production.
To assists in debugging please log client-request-id and request-id response headers and share with us.