Others have identified this as a machine with the wrong OS on it, an incompatible version of the OS on a Mac. However, I have found another cause.
To resolve the issue if it occurs, if it is caused by circumstances like mine, you would hold option+command+R then go into recovery and choose time machine and roll back. Then try again.
In my case I saw the Mac had updates. I selected to do the update which downloaded a 1.2gb file. During that download process I noticed that this mac could have Catalina installed (a new version of the OS). During this download the updater indicated that it wanted to reboot. I cancelled that. I let the Catalina downloader (no it hadn’t installed anything, it was just downloading) go for a short while. I then decided better of it and when the downloader was about 50% through downloading — no not installing — I chose to cancel it. I then restarted the Mac. That’s when I got the circle with the line through it. I rebooted. Same issue. I then decided to check the disk for errors. I rebooted holding option+command+r and ran the disk utility. That showed no issues. I rebooted again. No fix. I then started again with option+control+r. I’d noticed during the disk check that there was a time machine entry point of today’s date so I chose to go into time machine this reboot and selected to roll the mac back to that point. I rebooted and it worked. I was back to where I started. I then finished the initial update from the software updates, then after that rebooted and worked I decided to do the Catalina install. This began and seems to be running fine.
Be aware though if you use Catalina and have an older version of Microsoft Office that will no longer work. Maybe they have a patch for it now, but it will flag the software as incompatible and when you look at the icons it will be the icon with a circle and a line through it. So, be careful on your upgrades because you could cause yourself to disable certain software and thus cost you more money to resolve it.