Good evening JSquadman,
Welcome to the Apple Support Communities!
It sounds like you are seeing a prompt to update your macOS software to sync with your iPhone 7 running iOS 14.2.
If you want to sync your devices using a USB cable, then you may need to update the Mac.
You shouldn't have an issue with your older software if you upgrade to macOS Mojave 10.14.6.
macOS Catalina and the current macOS Big Sur require 64 bit applications and may not work with your older software.
Overview to syncing your Mac and your devices - macOS User Guide
Subscribe to iTunes Match - sync your iTunes library wirelessly with iTunes Match or Apple Music
How to get old versions of macOS
How to update the software on your Mac
If the issue persists, please reply to this thread and we'll do our best to help you!
Take care.