Time Machine cannot complete backup to read-only disk

I am getting a daily reminder that my Backup is not completed. When I searched for backup, I got the Time Machine as well as a Backup section (folder?). The Backup section had a red dot that when clicked on produced this notice

"Time Machine Couldn't Complete the backup to 'Back Up'. Files can't be copied on the backup disc because it is read only"

My Time machine has been working flawlessly for quite a long time and I cannot find anything I may have done to cause the problem. I did a complete reboot of the computer but that didn't help. I also ejected the flash drive but that did not help either. I tried a second flash drive to no avail. I also found a program for an update on my GPS that was floating around but even after ejecting that I am getting the same results.

What am I missing? I am running Tahoe as I cannot utilize anything newer due to my computer being an Intel chipped 27" iMac.