I am receiving this message when I'm trying to shut down my computer and I don't have any Finder windows open. What could be causing this message and how do I fix it so that I don't have to perform a hard shutdown?
Network errors can cause Finder to hang at Shutdown. Sometimes you can get around pressing the power button by quitting network manually before trying to restart.
This MacOS hint doesn't cover your exact situation, but may well provide a workaround for your hung shutdown: Making Finder BeachBalls go away without rebooting System
Turns out that using that to turn off Wi-Fi, then waiting for a bit, and turning Wi-Fi back on clears the Finder's beachball
It usually happens when it's trying to synchronize or back-up IOS devices. Sometimes it happens when it's doing mail.
What's bad is when the computer is doing its regularly scheduled shut-down and the message comes up, but I have already left. It stops the shutdown waiting for me to respond to the message and I don't discover it until the next morning, even though the cause of the message is no longer valid.
This is a real security issue.