Howdy avalon2kp,
It’s great to have you in the Apple Support Communities! I gather that you’re having issues connecting your iPhone 8 Plus to iTunes on your Mac. I’m happy to provide some info about this.
In order to connect your iPhone running iOS 13.3, you’ll need to run the latest version of iTunes. The latest version of iTunes requires macOS Mojave 10.14. To install that version of macOS, you’ll need one of these Mac computer models:Â
"-MacBook introduced in 2015 or later
-MacBook Air introduced in 2012 or later
-MacBook Pro introduced in 2012 or later
-Mac mini introduced in 2012 or later
-iMac introduced in 2012 or later
-iMac Pro (all models)
-Mac Pro introduced in 2013, plus mid-2010 or mid-2012 models   with a recommended Metal-capable graphics card."
How to upgrade to macOS Mojave
If your computer doesn't recognize your iPhone, iPad, or iPod
Best regards.