Once you have downloaded and installed iTunes 12.6.3 on your computer, you will be able to use the App Store section, download and install apps on your iPhone and install ringtones.
You can download iTunes 12.6.3 using the official download links below. For Mac only 64-bit version is available. On Windows iTunes 12.6.3 is available in both 32-bit and 64-bit versions. If you want to install iTunes 12.6.3 with App Store, you can rest assured that you will also not be bothered with constant upgrade notifications for the newer iTunes. Apple is doing this because businesses rely on iTunes to install apps on their devices, so by still making iTunes 12.6.3 available for download they are letting users get the missing functionality if they want to. Apparently Apple is still letting users download iTunes 12.6.3 for both Windows and Mac platforms, which allows users to search and download iOS apps on computer. If you are like most people and liked accessing the App Store on your Windows PC or Mac, then there’s a good news for you. You may also try the solution provided by VickyL19 in this thread. Restarting the device may help resolve the issue.
This means you cannot search and download apps for your iPhone or iPad using your desktop. The errors youre receiving are due to corrupt download or incomplete installation of a Windows Installer software. The latest versions of iTunes starting with iTunes 12.7 no longer allows users to use iOS App Store on their computer. In case you have not been paying attention, Apple has made a significant change in how iTunes works.