Hello Sharri,
Thank you for using Apple Support Communities! I understand that you're unable to manually sync music from your iMac to iPhone 6 using Finder unless you first remove all music on the iPhone. You've come to the right place for help.
Was the music currently on your iPhone synced from this specific Mac or a different computer? If it's synced from this Mac, do you have more than one iTunes library (Switch libraries with the Music app or Apple TV app in macOS Catalina)? The reason why I ask is that an iOS device can only sync to one iTunes library at a time. When syncing to a new computer or library, it is expected behavior that the synced content currently on an iOS device is erased and replaced with content from the current computer or media library.
You'll need to either switch to the iTunes library that the iPhone is synced with, or remove the music currently on the iPhone in that scenario. You can then select "Manually manage music, movies, and TV shows" under the General tab of the Finder window, and sync selected music to your iPhone after:
Use the Finder to sync your iPhone, iPad, or iPod with your computer - Apple Support
Sync music between your Mac and iPhone or iPad - Apple Support
Take care.