The installer is simply telling you it cannot run on your current OS because it was designed for High Sierra (10.13) or earlier. How to Get iTunes on macOS High Sierra (10.13)
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.
Download the real file.
If you are looking for the correct iTunes download for a High Sierra system, the information below will help you find the latest compatible versions and maintain your media library. Latest Official Versions for macOS 10.13
If you are running macOS High Sierra (10.13), iTunes is included with the system, but you may need to update it.
Are you trying to (like a newer iPhone)?