Install app on iOS without using iTunes

With the release of iTunes 12.7, Apple removed the ability to install apps on a connected iOS device by removing the App Store from iTunes.

I tried installing an older version of iTunes which still has the App Store, iTunes 12.6.3. However, when trying to log in, I need to authorize the computer, but when trying to sign in it gives me an error saying This action could not be completed. Try again. If I choose not to authorize the computer, it threatens to delete literally all of the apps on my device.

Is there another, better way to install an app on my iPhone?

Depending on your setup, you have two options.

• OPTION 1 - Use iTunes 12.6.3 instead. Refer to this answer for more details!
• OPTION 2 - Use Apple Configurator 2. Download and install Apple Configurator.

Since you're saying that iTunes 12.6.3 isn't working for you, than give Apple Configurator 2 a go. Apple Configurator 2 provides much more advanced features in terms of managing your iOS devices.

iMazing, which is a third-party app for iOS device management, can install apps to your device:
Here's a link to an article with more details.