Hi @tecnologianvs,
Welcome to the Auth0 Community!
Have you had a chance to decode your access token on jwt.io to verify the payload?
If your access token is not in the JWT format, it will not work against your API. It seems that you generated a JWT token when using it for your API but not when using the authorization code flow.
In this situation, you will want to Call Your API Using the Authorization Code Flow to get a JWT access token. After that, you can validate the access token using any existing middleware.
For more information, please refer to the following documentation:
Please let me know if you have any questions.
Thanks,
Rueben