I am using Scrivener both on Mac and on Windows platform. On the Mac, footnotes can be edited by clicking on them, which appears normal. On the PC, however, footnotes cannot be edited. Only when making the footnote the first time, I can write text in it and do changes. As soon as I go away from the footnote, no further changes can be made, and it is not possible to return to it and edit it. It has been like this for a long time (possibly all the time). To me this looks and tastes like a bug.
Inspector footnotes (and comments) should be editable by double-clicking into the note text. You can also select the note and then press Enter to switch to editing mode. Does none of that work for you?
Thank you. That solved it.