"Attempt to index local 'door' (a nil value)" Error. Can someone help me?

I am trying to make my door slide up when a Text button on a surfaceGui is clicked on. Here is my script:

    local door = game.Workspace.DoorSpawn7
    local newPos = door.Position + Vector3.new(0,40,0) -- the position the Part will go to
    local Time = 10 -- the time that the script will take to move the part
    local Increment = 0.5 -- the Part will move 0.5 studs each time it moves
    local Debounce = false
    local Diff = newPos - door.Position -- the difference between the two positions
    local Mag = Diff.magnitude -- the distance between the two parts
    local Direction = CFrame.new(door.Position, newPos).lookVector
    local TxtBox = door.SurfaceGui.TextLabel

function onTouch(door) -- The function that runs when the button is clicked.
    if Debounce then return end
    Debounce = true
    TxtBox = 'Door Is Now Openning...'
        for n = 0, Mag, Increment do
            door.CFrame = door.CFrame + (Direction * Increment)
            wait( (Time/Mag) * Increment )
        end

    Debounce = false
end 

script.Parent.MouseButton1Click:connect(onTouch)

I'm not sure what the error is. I'm new to LUA but have experience with other scripting languages (javascript.) Also, I looked up this error, and found a Related Question similar to the problem i'm having. What I understood from the answer and other commenters who answered to the error. Is that for example, you can't change the meaning of a variable and then try to get the original by calling it back but instead get the changed one, probably not the best example but hopefully this was enough information to help solve the problem and know where I stand in my understanding of this error. Thank you for your help in advance, Addictedroblox1414.