Recovering the ability to install an iOS app from macOS
You must have iTunes 12.7 or later installed on your Mac.

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 with App Store built-in (iTunes 12.6.3) via the Apple Support article, Deploy apps in a business environment with iTunes.