Thanks you so much for responding. I did know it only works for the newly published posts since that is the reason I first asked you for help. With your help I worked through that problem. It worked for a couple weeks, and now the same glitch. I suspect that it is something Facebook is doing, but I am not sure.
I followed your instructions and it appears to have worked, but I don't know for sure since my post is still in draft form, but it at least allows me to tick the box while before it did not.
Anyway, thank you so much for your patience and help.
M.