This is the most common point of failure for Mac users. Apple intentionally blocked the installer for iTunes 12.6.5 on Mojave because they intended to replace iTunes with the new Music/TV/Device apps.

💡 If you are on a modern Mac (macOS Big Sur or newer), consider using Apple Configurator 2 from the Mac App Store as a modern alternative for managing iOS apps. If you'd like, I can help you with:

: To ensure it stays on 12.6.5, go to Edit > Preferences > Advanced and uncheck "Check for new software updates automatically."