If open = false then part.Transparency = 0 else part.Transparency = 1 not working?

This should work, you need to use an elseif statement, because what you are using right now is checking to see if "open" is true, and changing it to false, then checking if it is false, and changing it to true, so it just opens and closes immediately

open = true

function onClicked(click)
    if open == true then
        open = false
        script.Parent.Parent.Curtain1.Transparency = 1
    elseif open == false then
        open = true
        script.Parent.Parent.Curtain1.Transparency =0
    end
end
script.Parent.ClickDetector.MouseClick:connect(onClicked)