iMac Software Updates Won't Complete

I get notices that new software updates are available for my iMac. But when I try to install them, they never complete. The progress bar goes slowly to the right, up to the end, but "hangs" there. Then I get a panel that says iTunes must be closed, even though it is closed. I can't get rid of either notice and must restart the iMac. This has happened several times. Any suggestions?

iTunes is closed by actually selecting iTunes in the dock or menu bar and selecting quit?
If you are using the red dot at the top left you are not closing the app.

In fact hitting the red dot closes no applications at all, merely the window. Always use the Quit option.

harryb2448 said:

In fact hitting the red dot closes no applications at all, merely the window.

Click to expand...

This is not correct. Sadly, Apple is not consistent about this.

What you said is MOSTLY true, but (for example) pressing the close button on System Preferences does, in fact, quit it (and there are other examples).

Your advice, of course, is nonetheless the best practice and the user should take heed of it.