hello my light switch code isn't working please help?

I'm not sure if this is gonna fix it, but I think you might have to specify what "switch" actually does

If switch == 0 then switch == 1

You have the right idea but you have made a few errors, I fixed them all, hopefully this helps. And make sure it's a script and not localscript

local Clickdetector = script.Parent.ClickDetector
local Switch = YourValueHere.Value -- create a Value in ReplicatedStorage

Clickdetector.MouseClick:Connect(function(click)
    if Switch == 0 then
        game.Workspace.MeshPart.PointLight.Range = 0
        Switch = 1
    elseif Switch == 1 then
        game.Workspace.MeshPart.PointLight.Range = 12 
        switch = 0
    end
end)

To start off, please format your code. I've done it for you, thank me later.

local Clickdetector = script.Parent.ClickDetector

local Switch = 0

Clickdetector.MouseClick:Connect(function(PlayerWhoClicked)
    if Switch == 0 then
        game.Workspace.MeshPart.PointLight.Range = 0
        local Switch = Switch + 1
    elseif Switch == 1 then
        game.Workspace.MeshPart.PointLight.Range = 12
        local switch = switch - 1
    end
end)

There are about 2 problems: First, the major problem, is that you;'re redefining the variable Switch. Saying local X = 5 will create a new variable named X, and limits it to that scope. Saying local Switch = Switch + 1 will create a new variable called Switch, and set it to the desired value (Switch + 1), but it will not overwrite the Switch variable that you're trying to overwrite.

To give an example of this:

local X = 5

do
    local X = 10
    print(X)
end

print(X)

Will print 10, 5

The simple fix is to simply remove the local before the variable declaration thingy:

local Clickdetector = script.Parent.ClickDetector

local Switch = 0

Clickdetector.MouseClick:Connect(function(PlayerWhoClicked)
    if Switch == 0 then
        game.Workspace.MeshPart.PointLight.Range = 0
        Switch = Switch + 1
    elseif Switch == 1 then
        game.Workspace.MeshPart.PointLight.Range = 12
        switch = switch - 1
    end
end)

The second problem is that on line 11, you didn't start Switch with a capital, while you did start the first Switch variable with a capital. To fix this:

local Clickdetector = script.Parent.ClickDetector

local Switch = 0

Clickdetector.MouseClick:Connect(function(PlayerWhoClicked)
    if Switch == 0 then
        game.Workspace.MeshPart.PointLight.Range = 0
        Switch = Switch + 1
    elseif Switch == 1 then
        game.Workspace.MeshPart.PointLight.Range = 12
        Switch = Switch - 1
    end
end)

And then your code will work. Bit weird that you're incrementing and decrementing Switch by 1, but it definitely works.