In the URL entered into your Auth0 app’s callback URL, you need to replace those placeholders with the values they represent.
So instead of entering this in Auth0 app settings:
{YOUR_BUNDLE_IDENTIFIER}://${YOUR_AUTH0_DOMAIN}/ios/{YOUR_BUNDLE_IDENTIFIER}/callback
You should enter something like this:
org.reactjs.native.example.MyApp://MyAccount.auth0.com/ios/org.reactjs.native.example.MyApp/callback
You can find the correct value for ${YOUR_AUTH0_DOMAIN} in the Domain field in your Auth0 app’s Settings page.
I am not exactly sure where to get the value for {YOUR_BUNDLE_IDENTIFIER} yet as I am just getting started with React Native. If you created your app with ‘react-native init MyApp’ then I suspect it might be org.reactjs.native.example.MyApp (obviously replacing ‘MyApp’ with whatever name you specified.
Please let me know if this helps.