Product Page

iTunes 12.7 published on September 12, 2017 and later versions remove the ability to download and manage iOS applications. Because of this change, it’s no longer possible for Crunch to get access to iOS application archives (.ipa files) downloaded using iTunes.

The App Store has been entirely eliminated in iTunes 12.7, as has the ringtones section of iTunes.

Apple Releases iTunes 12.7 With Major Changes, Including No Built-In App Store MacRumors

Here are some alternatives to download your purchased iOS applications from your Mac:

Install iTunes 12.6.3

Apple provides a special version of iTunes, actually tagged as version 12.6.3, which gives the ability to download and manage iOS applications.

You won’t be prompted to download new versions of iTunes after you install this version.

Downloading iOS apps without iTunes

iMazing is a is a third party app that allows, among other things, to redownload purchased iOS applications.

iMazing is not affiliated with Crunch in any way.

Go deeper

Finally, if you want to get more information, you can read this thread on stackexchange hinting various solutions to download .ipa files from a Mac.