No entry sign on start up
I’ve been running Catalina on a late 2012 iMac for some time. During the last update I was informed that the update could not be completed because of a kernel cache issue. On restarting the Mac I was presented with the no entry sign suggesting that the OS was not compatible. I opened in the Mac in Recovery mode, repaired the disk and tried again to reload the OS (automatically it tried to download Catalina). Again installation failed due to a kernel cache issue. I opened again in Recovery mode and this time erased the disk opted for a clean install. Catalina loaded ok with the latest update and all seemed well. However on trying to access some files on Lightroom I was presented with the wheel of doom and the Mac then crashed. It would not restart and I was presented again with the no entry screen. Clearly Catalina does work on this Mac configuration so does anyone have any idea as to what is going wrong?
iMac 27″, macOS 10.15