Mac won't shutdown as error 'install in progress' keeps showing

Are you being provided any clues on what app is attempting to be installed that you are getting this message for?

By "tool bar" I'm assuming you are referring to the Apple Dock, which by default would be at the bottom of the macOS Deskop. If that's the case, chances are this has to do with the macOS Finder.

I suggest two things:

1. Hold down the power button until your Mac shuts down completely. Then perform a Safe Mode boot. After it boots up in this mode, see if this issue continues. If it does, continue with the next step.
2. Power down the Mac, and then again, boot up in Safe Mode. While in this mode, create a new user account. Restart the Mac in normal mode, but this time, log in with the new user account. Does this issue still persist?

Try holding the power button in until it shuts down.

Use Force Quit to shut down all applications. Click the Apple symbol in the upper left of the display, select Force Quit and Force Quit all apps, this will work on all apps except Finder.

You should not shutdown during the installation of an update or upgrade.

You should leave the iMac on until the update or upgrade has completed.