iTunes on Macbook Pro doesn't recognize my iPhone 6s when connecting to USB

So I recently tried to plug my iPhone 6s into my 2012 MacBook Pro to sync my new music to my iPhone. When I connected my iPhone to my Mac and then opened iTunes, a notification saying "To use your iPhone with iTunes, you need to install OS X 10.11.4 (or later)". So I updated my MacBook to macOS High Sierra 10.13.4, thinking that I needed to update to the latest version available to my computer. Then when I tried to plug my iPhone again to my Mac, no notification came up but neither did the iPhone icon that would pop up in iTunes when I wanted to add more music to my iPhone from my mac. So now I don't know how I can sync my music from my mac to my iPhone because it does not recognize it. I also don't know if I need to update my Mac to something newer or not because it does not notify me of any computer updates anymore. Please help!!