Welcome to Auth0 Community. It’s great to have you on board.
It sounds like there’ll be an issue with the request that you’re making if you’re getting a 400. If you have a native app I’m assuming we cannot review the browser dev tools to have a look at the request that’s being sent? Maybe you can put some logging in to the backend code, analyse the /authorize request that’s being sent and ensure it looks something like this along with the correct parameters https://auth0.com/docs/get-started/authentication-and-authorization-flow/add-login-using-the-authorization-code-flow-with-pkce#authorization-url-example
It might also be worth taking the request that you’re making and send it across via a curl command to help troubleshoot the issue.
Feel free to let us know if you still have issues with this.
Regards.