Hi @sam77
Welcome to the Auth0 Community!
Thank you for posting your question; due to security reasons (more about that here → Security Risks of Using Localhost for Callback URL ), We are not allowing the localhost for the application login uri. To workaround this, replace the localhost with 127.0.0.1 → Application Login URI field payload validation error for https://localhost:3000/authorize - #20 by tunedev
Thanks
Dawid