Why won't my boolvalue change?

The issue here is that you are defining IsOn twice at the start of the script. The second declaration of the variable is what the variable becomes since scripts run top to bottom. Get rid of line 4 and edit the actual IsOn BoolValue object to be true at first.

local screen = script.Parent.Parent.Screen
local light = script.Parent.Parent.Light.SurfaceLight
local IsOn = script.Parent.IsOn.Value

script.Parent.ClickDetector.MouseClick:Connect(function()
    if IsOn then
        IsOn=false
        light.Enabled = false
        screen.BrickColor = BrickColor.Black()
        screen.Material = Enum.Material.SmoothPlastic
    elseif not IsOn then
        IsOn=true
        light.Enabled = true
        screen.BrickColor = BrickColor.Blue()
        screen.Material = Enum.Material.Neon
    end
end)