The meaning of the error message is that you’re using a local adjustment tool like Gradient or Adjustment Brush but all the sliders are set to zero so applying the local adjustment won’t have any effect. This wouldn’t have anything to do with installing El Capitan, I don’t think, and is just a coincidence. See here:
If you are getting this error when moving other sliders, for example Exposure in the Basic tab then maybe something else is occurring and the message is wrong. El Capitan does have problems with permissions being read-only after the upgrade, so maybe that’s also what is happening, especially if your images are on an external drive.
Yup, that was the problem.
You are literally an angel...
Thank you so much!
If Google is a deity, perhaps. I merely Googled the error message and the top link or two was the discussion I linked to.