I have an iPhone 7 plus with iOS 12.0.1 and Mac OS 10.12.6 Sierra and now my iPhone is not recognized by my iTunes to backup.

Looks like your phone is on iOS 12 but your Mac is on macOS Sierra. You need to update your macOS to update iTunes, and once iTunes is up to date you’ll be good: http://support.apple.com/en-us/ht209142

I literally spent like 6 hours total when I got my iPhone XS max because it wouldn't be recognized in iTunes. I tried changing cords. I tried plugging into different USB ports. The phone would charge, but would not pop in iTunes. I tried plugging in my previous iPhone 7 plus, and it would charge and would pop up in iTunes and I was able to back it up, but for some reason the XS would not. I tried chatting apple support on multiple occasions, and I read so many blog posts with steps to take; restarting all devices and trying to uninstall iTunes and etc. All apple chat would tell me is "update to the newest mac OS," even though I had said multiple times that I didn't want to.

Finally, one apple worker was able to give me an answer that worked, and it was so easy.... all I had to do was go to iTunes - Upgrade to Get iTunes Now - Apple and click download iTunes 12.8 and follow the steps, and my phone was able to be detected!!! Yes I already had iTunes, but the lady said that sometimes I guess theres a glitch in the coding and by reinstalling it through apple it's able to be compatible! SO easy, I wish someone had told me ahead of time. Hope this works for you!

My problem was solved by re-installing itunes version 12.8

Got the same issue just now but mine is on windows 10 and iphone 7. argh!

resolved mine, follow this link: Remove and reinstall iTunes and related software components for Windows 7 and later - Apple Support

I have the same problem....Pls help