Q: Error 404 for an app download, and it won't go away. How do I stop it?

Hi,

I tried to download slack, as you can see in the picture below. For about a month, I've been getting the error in the top right of the screen shot.  It's really annoying, as I can't seem to cancel the download, and the itunes app always bobs up and down to tell me there's an error 404 with the download, but it doesn't let me cancel the download!

Please help.

Thanks,

Richard

Mac 10.10.5