You said it was plugged into a tv - it won’t do it on an external display.
The option does not appear even if I use an external monitor.
Do you also have a ZEN 3 CPU?
It WON’T appear until you remove the external monitor.
When I remove mine and go back to the settings - it’s there
I don’t use Gnome now, but I remember that I found a solution to the screen dimming problem, which I wrote on discourse.gnome.org. I forgot that LightDM doesn’t, but GDM supports screensaver. I replaced the former with the latter, and now the screen dimming works.
Thank you! Will se if I can find something in there?
Here is the original post at discourse.gnome.org. It worked for me, although since then I no longer use Gnome, I switched to the “opponent” Although I think you wrote from an external monitor, I don’t know if this solution will work there, hopefully it will.