Q: I have a new iPhone 6 s. Im trying to build it / restore.

You need to get the latest version of iTunes. Get the latest version of iTunes - Apple Support

What version of iTunes is running on your Mac? What version of OS X

is on your Mac?

Just to be sure the latest version of iTunes on your Mac should be iTunes 12.4. The phone should be running iOS 9.3.2. Are those what you are running? If not you could have an older OS on the Mac that is keeping your from getting iTunes 12.4

My guess is that you are running OS X Lion (10.7) or earlier, which is not compatible with the latest version of iTunes. You need to upgrade OS X to update to a newer iTunes version.

https://itunes.apple.com/us/app/os-x-el-capitan/id1018109117?mt=12

Yes, they are too old. Here are the requirements for El Capitan which should let you run iTunes 12.4

Upgrade to OS X El Capitan - Apple Support

No problem....