Hello Art Vandelay100,
It sounds like the Photos app is not allowing you to properly add a Caption or Keyword to photos. Being able to add this information can be crucial in how your organize your photos so we want to help get this sorted.
To start, we want to confirm that outdated software isn’t causing this. Make sure that you’re backed up and running the latest version of Big Sur.
For a full breakdown on using Time Machine to back up your machine, visit: Back up your Mac with Time Machine
For a full breakdown on updating to the latest version of macOS Big Sur, visit: How to update the software on your Mac
If you’re already updated, or the behavior continues after updating, we next recommend trying to add a Caption or Keyboard while the computer is in safe mode to see if the same thing occurs. To launch into safe mode, please follow the steps here: Start up your Mac in safe mode
If the behavior doesn’t occur in safe mode but returns under a normal launch, make sure you have no automatic items that could be causing the behavior you’re experiencing in Photos. For a breakdown on checking your automatic items, visit: Open items automatically when you log in on Mac
We hope that helps. Take care.