If you see an error message like https://YOURFUSIONAUTHSERVER/en?error=invalid_request&error_reason=missing_code_challenge&[…]The+request+is+missing+a+required+parameter%3A+code_challenge
it is because you have required PKCE for your application, but have not provided those parameters when attempting a login/registration. The links that we autogenerate in the app will not have a code_challenge by design, because those are dynamic for each request.
You can learn more about PKCE and turning it off on the applications core concepts page.