Hi @shibui,
Welcome to the Auth0 Community!
access_denied happens when Auth0 cannot issue a token (Auth0 refuses the authorization).
For a demonstration of this issue, refer to the following video.
If receiving an access_denied error, first check the rules. Another possible reason is that an incorrect CLIENT_SECRET or audience is passed in.
I recommend capturing this flow in a HAR file and analyzing the result. For guidance on how to create a HAR file, refer to the following doc:
Also, please check these articles for solutions:
Thanks,
Timotei