What did i do wrong?

Well, I think the problem is that you're expecting something to happen. If you just change the value of on when the button is clicked, then you won't notice a change. The only thing that will change is the value of open.

Try changing your script to this, this will make the light on or off.

on = true 
local bulb1 = game.Workspace.Light_Model.Light.PointLight.Enabled

function onClick() 
if on == true then 
on = false
bulb1 = false
elseif on == false then
on = true
bulb1 = true
else
print("An error has occurred with the light script.")
end 
end

script.Parent.MouseClick:connect(onClick) --also, you don't connect to the ClickDetector, you connect to the part that has the ClickDetector in it.

In the future, make sure your code is in a code block. It's that little blue dot that says 'Lua' in it, like this:

on = true 
local bulb1 = game.Workspace.Light_Model.Light.PointLight.Enabled

function onClick()
    if bulb1 == true then
        on = false 
    elseif bulb1 == false then
        on = true
    end 
end

script.Parent.ClickDetector.MouseClick:connect(onClick)

I will assume that you want to turn the light on and off when clicked. This is simple to do, and just takes a little rearranging of your code.

local on = true 
local bulb1 = game.Workspace.Light_Model.Light.PointLight 

Using local variables for each makes it easier to read and less laggy. Also notice that I set bulb1 to the pointlight, not the pointlight's Enabled property. This is because when you set a variable to an instance's property, the variable becomes whatever the property is at the time the variable was defined. It will not change if the property changes, and you can not change the property by changing the variable.

if on == true then
    on = false 
    bulb1.Enabled = false   
elseif on == false then
    on = true
    bulb1.Enabled = false   
end 

Read this a few times, I hope it makes sense, because I'm not sure how to explain it further. If on is true, we turn the light off. If on is false, we turn the light on. Simple.

Hope I helped!