Let us assume you have a pristine copy of macOS 10.13.99 (or rather, 10.13.6) and need to install iTunes from scratch.
Do not search for “iTunes 13.” It does not exist. Do not download from “iTunes.com” pop-ups—those are scams.
Do you need assistance updating your Mac to the actual final version of ? Share public link
Ensure you have installed the Security Update 2020-006 for High Sierra. Run the Mac App Store software update tool to grab all final security patches. iPhone Not Connecting to iTunes