software updates

Perhaps we need to clear up exactly which versions of iTunes and OS X you have installed? The current version of High Sierra is 10.13.6. The last version of Mavericks is 10.9.5. I suspect you have the latter and not the former after all. What model is your MacBook Air? See How to upgrade to OS X El Capitan - Apple Support for details on updating your OS to something more recent that can in turn install a more recent version of iTunes.

tt2

I'm not at my Mac right now but I think iTunes > About iTunes should show your version number.

For iOS 12 you need iTunes 12.8.0.150 or better running on OS X 10.11.4 or better. See this collection of iTunes for Mac installers - Apple Community if you need to upgrade/reinstall.

tt2

See the Update your device using iTunes section of Update your iPhone, iPad, or iPod touch - Apple Support.

tt2

Are you getting an update notice on the iPad? What model number is the iPad? A_ _ _ _? If this is a first generation iPad mini, it cannot update past iOS 9. That last iOS 9 update is 9.3.6

Update your iPhone, iPad, or iPod touch - Apple Support

What macOS are you using and what version of iTunes are you using?

Redundant post removed.

tt2