There is no official version of macOS numbered . The final and most current update for macOS High Sierra is 10.13.6 .

For users on macOS 10.13.6, the latest version of iTunes available is .

The error message requiring is a frequent point of confusion for modern Mac users. It typically appears when trying to install a standalone iTunes installer on a system running macOS Catalina (10.15) or later. The Core Issue: Why iTunes "Disappeared"

If you are running macOS High Sierra (10.13), iTunes is included with the system, but you may need to update it.

Installation & download considerations

Note: macOS version 10.13.99 does not exist as an official Apple release. The final version of macOS High Sierra is 10.13.6. This article addresses the user intent behind the search (finding the last compatible iTunes for older macOS versions) and corrects the version number while providing safe download solutions.

Even when downloading the correct iTunes 12.8.3 for macOS 10.13.6, you might encounter issues. Here is the troubleshooting guide: