The App Store has disappeared from iTunes since the 12.7 update but is still accessible from a computer, provided you reinstall the correct version of the software.
Since the last official update of iTunes to version 12.7 in September, Apple has simplified its multimedia software by removing all the section dedicated to managing applications. If the barge can no longer explore the shelves of the App Store from his computer, he also finds himself deprived of the possibility of managing his applications.
At first glance, this does not seem to bother users who have become accustomed to using only the iPhone and iPad to find and install applications. But Apple backpedaled by unveiling a new version of iTunes, prior to 12.7. Despite the existence of tools dedicated to managing a fleet of iOS devices – including the volume purchase program and Apple Configurator – some professionals may still need iTunes to install applications on iPhones or iPads explains Apple in a support note.
Whether you are a professional or not, you can proceed to the installation of iTunes 12.6.3 to regain access to the App Store as well as to the application management panel, from your PC or your Mac.
To do this, you need to download iTunes 12.6.3 on your machine and run the installation over iTunes 12.7.

Download iTunes 12.6.3 – 32 bits for Windows (Free)
Download iTunes 12.6.3 – 64 bits for Windows (Free)
Download iTunes 12.6.3 for Mac (Free)
When opening iTunes 12.6.3 for the first time, the program should display an error message stating that the file “iTunes Library.itl” cannot be read by the program, the program having been converted for a later version of the program. ‘iTunes.

Here are the steps to take to remedy the problem
1. Go to the Music> iTunes directory on your machine. Locate the iTunes Librabry.itl file, and delete it.
2. Go to the Music> iTunes> Previous iTunes Libraries directory to copy the most recent version of the file named with the architecture “iTunes Librabry aaaa-mm-dd.itl”.
3. Paste the file into Music> iTunes and simply rename it iTunes Librabry.itl.
After opening iTunes, to find the applications pane, click on the drop-down menu at the top left, where “Music” is selected by default, then choose “Applications”.
You should find the panel dedicated to applications from where it is possible to launch the synchronization of applications on your iOS devices, but also to access the App Store.