If you updated to iTunes 12.7 and want to go back, here is the fix! Apple has released iTunes 12.6.3, which reintroduces the ability to download App Store apps and ringtones from within the iTunes desktop software.
If you’ve already installed a newer version of iTunes, you can download this version of iTunes on your Mac, PC 32-bit or PC 64-bit and run the installer. After installation is complete, you can continue to deploy apps with iTunes.
You won’t be prompted to download new versions of iTunes after you install this version.
Apple is making this version of iTunes available because “certain business partners might still need to use iTunes to install apps”, but the download is basically available to anyone looking to reinstate the functionality that was removed in iTunes 12.7.
The Installer packages for Mac and PC (32-bit and 64-bit) are available from Apple on their support page.
After installing iTunes 12.6.3, you will need to rebuild your iTunes Library. To do this, hold down the Option key when the app launches and in the dialog window that appears choose “Create Library…”. Give the new library a name that’s different to your existing library, and then migrate over your albums using the Add to Library… option under the Files menu.
This iTunes 12.6.3 version includes compatibility with iOS 11, iPhone 8, iPhone 8 Plus, and iPhone X. Apple will not provide technical support for this version.