There have been reported issues of users having difficulty installing apps on the iPhone X. Here is a possible issue you could encounter and how to circumvent it when trying to download the Knit Sleep Assistant app.
Double Click to Install message: If you see a "Double Click to Install" message, double-click the button on the side of your phone. This will activate Face ID, which is used to confirm the installation of the app.
Then you can install your Knit camera.