The Finder can’t quit because an operation is still in progress on an iOS device.

This error appears when trying to shutdown or restart my Mac.

This can happen when an iOS device hasn't finished syncing correctly. To fix this we need to kill a process called Usbmux.

Open Terminal. Applications->Utilities->Terminal.

Type sudo killall -9 usbmuxd and press Enter.

Type in your administrator password and press Enter.

You can now shutdown your Mac.