To use your iPhone with iTunes, you need to install OS X 10.11.4 (or later).
My even older Mac Pro is running 10.10.5 and Software Update doesn't give an option for an upgrade to 10.11.4. That's the highest OS (so "or later" isn't possible) this Mac Pro is capable of running (even then, with a few features not possible).[/i]
What can I do--just bite the torpedo and upgrade to a later model Mac Pro?