Photos won't let me edit anything / Same problem on Sierra So (my mom) is trying to edit photos in OS X El Capitan's Photos app and can't seem to do so, all the options are just greyed out. This is true of all the photos in the app.

I looked around online and found a couple of people who had the same issue but no-one ever seemed to have solved it.
I was having the same problem.  I do notice a (barely visible against light background) exclamation mark in a circle in the lower right corner of the photo, which indicates that my disk is full. (I'm using apple cloud storage.)  So that could be it.