How do I sync apps to my iPhone with iTunes 12.7 or later?
With the release of iTunes 12.7, Apple removed the ability to browse, download or install iOS apps via iTunes for macOS/Windows.

In case you wish to browse App Store on your Mac/Windows PC, you can still download the last version of iTunes (iTunes 12.6.3) with App Store built-in via the Apple Support article, Deploy apps in a business environment with iTunes.