To run the sample follow these steps:

com.auth0.ionic://__AUTH0_NAMESPACE__/cordova/com.auth0.ionic/callback

com.auth0.ionic://__AUTH0_NAMESPACE__/cordova/com.auth0.ionic/callback

http://localhost, http://localhost:8100

iOS

ionic://localhost, http://localhost:8100

Ensure that Ionic 4, Cordova and make sure native-run are installed.
Check that mobile development environments for Android and iOS are setup correctly.
Make sure Node.JS LTS is installed and execute the following commands in the sample directory:

npm install
# to test it in iOS
ionic cordova run ios --livereload
# to test it in Android
ionic cordova run android --livereload

What technology are you using for your web app?