The reason you’re getting an error is because you’re using an App access token. You need to generate a user access token and permit it the scope to access your extension analytics.
As for documentation, the docs do state Since app access tokens are not associated with a user, they cannot be used with endpoints that require user authentication., and the endpoint you’re trying to access requires your user account that owns those extensions to grant permission in the form a User Access Token with the analytics:read:extensions scope. There are also numerous topics on the subject on these forums explaining the differences.