Restarting
The finder can’t quit because an operation is still in progress on an IOS device
iMac 27″, macOS 10.15
Assuming the apparent sync operation never finishes I think it's a macOS or iOS bug. Until Apple fixes it eject the iPhone from a Finder Sidebar, or force your Mac to restart by pressing and holding its power button until it shuts down.
Apple may have fixed it by now. Make sure both macOS and iOS are updated.