Hello @BlitzkriegSoftware, Welcome to the Auth0 Community!
This specific error is for Application Login URI field.
If you click on the Learn More link below the Application Login URI field, You will be redirected to:
There it is mentioned the The login_url should point to a route in the application that ends up redirecting to Auth0’s /authorize endpoint, e.g. https://mycompany.org/login . Note that it requires https and it cannot point to localhost .
For this particular field only you cannot use localhost.
Please let me know if this helps!