Hello there, reimeang.
It sounds like you are being given the message that you need a newer version of iTunes when you are trying to sync your iPhone 6s Plus to your Mac. The reason you are seeing that message is because the iPhone 6s Plus requires iTunes 12.3 or later. The current version of version of iTunes (12.3.2) has a requirement of OS X 10.8.5 or later. Since you are running Mac OS X 10.7.5 you are unable to upgrade to the latest version of iTunes that the iPhone 6s Plus requires.
iPhone 6 - Technical Specifications
iTunes Download and System Requirements
In order to be able to sync your iPhone to your computer, you will need to update the Operating System to Yosemite or El Capitan. El Capitan is currently available in the Mac App Store at the following link:
OS X El Capitan - Mac App Store
Thanks for reaching out to Apple Support Communities.
Kind Regards.