File google-services.json is missing

Root directory is your project directory which you use to create your Ionic app.
It means if you create your app in directory as ‘G:/Projects/XYZApp/’ then your root directory will be ‘G:/Projects/XYZApp/’. Copy your file here and change the config.xml as answered by HugoHeneault.
A workaround would be to copy google-services.json file into the directory ‘G:/Projects/XYZApp/platforms/android/app/’, but it will require you to copy the file into this directory each time you build your app. So the option to add it in your root directory and change the preference of app in config.xml is preferable.