Why isn't the part detecting that it's being touched?

On line 9, in your first script, you are firing the remote event to a client. however, u didnt pass any valid arguments, so no client receives it. As the touch function would fetch the character, which is on the server, the remote event wouldn't be needed. Instead to do this:

--In StarterPlayerScripts
local debounce  = false


workspace.AmbientChanger.Touched:Connect(function(hit)
if debounce == false then debounce = true
if hit.Parent == game.Players.LocalPlayer.Character then
if game.Lighting.FogEnd == 100000 then --Make sure other events wouldn't change this prior!

        for i = -1000, 100000, 500 do
        game.Lighting.FogEnd = i
        game.Lighting.FogColor = Color3.fromRGB(255, 255, 255) --RGB, not New, is out of 255
        end

    else

        for i = 1000, 500, 100000 do --Change the third argument? Would give off an error.
        game.Lighting.FogEnd = i

        end
end
    end
end
debounce = false
    end)

Now, let me explain this, as you're trying to change the lighting on the client, and you need to know if that same client touches the part, it should all be done on a local script, eliminating the need of a local script. Rather, you should place the touched function in the same script. As well as checking if there is a humanoid, I would also check if on of the parts on the client's characters set it off. If so, it'll continue on with the rest of your code.

All in all, if you need to change stuff on the client with :Touched events, just combine them in a localscript, while adding some precautions so it'll work.

Hope this helps!