What causes "The Finder can't quit because some operations are still in progress" error message?

Solution 1:

Here's another way to Force Quit an application:

1. Click the Apple Menu
2. Hold Shift, and notice that the 'Force Quit...' menu item has changed to 'Force Quit [frontmost app name].'
3. Select 'Force Quit [frontmost app name]' to force quit the Application.

Solution 2:

This usually comes up when the Finder is doing a long copy or emptying the Trash. Are you sure you don't have a progress window minimized to the Dock or on a different Space?

Solution 3:

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