The immediate solution for this is to update your Machine’s iTunes version to 12.5.1, in case, it requires an updated OS X / macOS version to run. Exceptions are  Windows 7,8, 10.

You are now required to upgrade. You have to buy a new computer to sync to that phone, or use BootCamp and install a Windows release.

You’re not the first to come here with this situation and probably not the last.