Remember when you could reboot your computer without rebooting your phone first?
I’m not even kidding: I needed to reboot my Mac because I was unable to navigate character by character using the arrow keys when composing new iMessages in Big Sur, but Finder refused to quit during the reboot with the above error message. It was still syncing my iPhone. (Remember when we thought the iTunes rewrite would be a good thing? Good Times!) I tried quite a few things on both devices, but was unable to cancel said sync in any other way than to reboot the phone.