it is because you are trying to install a standalone version of on a Mac running macOS Catalina (10.15) Apple Support Community
There is no actual macOS version "10.13.99"; this is a version check limit set by Apple to prevent the installation of iTunes on systems where it has been replaced. Apple Support Community Why You Can't Download iTunes Starting with macOS Catalina (10.15), Apple split iTunes itunes macos 10.13.99 download
The Digital Architect: The Legacy of iTunes on macOS High Sierra it is because you are trying to install
: Open the Apple Music app already installed on your Mac. itunes macos 10.13.99 download