trying to make sort of a stop light system, but it won't work?

I believe you are not calling the function, as well as the function command is quite outdated. This is how I would go about it, since I don't know what you want it to be triggered on, I'm using touch.

local purpleLight = game.Workspace.Model.Purple.PointLight
local yellowLight = game.Workspace.Model.Yellow.PointLight
local greenLight = game.Workspace.Model.Green.PointLight
local blueLight = game.Workspace.Model.Blue.PointLight
local Trigger = game.Workspace.Trigger

Trigger.Touched:Connect(function(hit)

if hit.Parent:FindFirstChild('Humanoid') then

    purpleLight.Enabled = true

    wait(1)

    purpleLight.Enabled = false

    blueLight.Enabled = true

    wait(1)

    blueLight.Enabled = false

    yellowLight.Enabled = true

    wait (1)

    yellowLight.Enabled = false

    greenLight.Enabled = true

    wait(1)

    greenLight.Enabled = false

    end
end)

Let me know if this works!

Try adding "triggert()" at the end of the script.