Hi Katyjoe1, can you please tell me what Xfinity app you're trying to download? Have you tried downloading the app on another device other than your IPhone X? Is there an error message that comes up when you try and download the app?
If you see “Double Click to Install” on your iPhone X, all you have to do is double-click the Side button. This will activate Face ID, which is used to confirm the installation of the app.