It is not saying install iTunes on your phone. That is impossible because phones cannot run iTunes. It is saying you have to upgrade your iTunes version on your computer. You have apparently upgraded iOS on the phone to iOS9 which requires iTunes 12.3 which in turn requires a computer running OSX 10.8.5 or higher. Your computer is running OSX 10.6.8 (so it says in your profile). You can either buy a download code for OSX 10.8 of you can install El Capital for free. You may need to install more RAM in your computer.