Repeat until isn't working properly?

If you want the door to slide on its own without a trigger and then stop when it reaches the end, this could possibly work.

1   posi = -31.7
2   posi2 = 4.81
3   posi3 = -57.215
4    
5   while true do
7       if posi < 40.7 then
8       wait(0.1)
9       posi = posi + 1
10      game.Workspace.Part.Position = Vector3.new(posi, posi2, posi3)
12      end
11  end

Instead of using while, repeat loops, use for i and lerp.

It's simple. Duplicate the door part two times and name the first one StartPart and the second FinishPart. You should have three door parts but all different names. Position the StartPart and the FinishPart. (don't forget to make the duplicate parts invisible and cancollide false)

local DoorPart = workspace.Door --(change your door part's name to 'Door')
local StartPart, FinishPart = workspace.StartPart, workspace.FinishPart

Now that you set the variables, get both start and finish parts CFrame.

local DoorPart = workspace.Door --(change your door part's name to 'Door')
local StartPart, FinishPart = workspace.StartPart, workspace.FinishPart

local StartPos, FinishPos = StartPart.CFrame, FinishPos.CFrame

Next, create a function and use the for i loop.

local DoorPart = workspace.Door --(change your door part's name to 'Door')
local StartPart, FinishPart = workspace.StartPart, workspace.FinishPart

local StartPos, FinishPos = StartPart.CFrame, FinishPos.CFrame

local Tick = .1 --higher value will make the door transition look rough, really low values take too long. Suggest keeping this  greater than .1 and less than .3

function openDoor()
    for i = 0, 1, Tick do 
        DoorPart.CFrame = DoorPart.CFrame:lerp(FinishPos, i) --transitions the door part to the finish part's cframe
        wait() --don't add a number in wait() it makes things look choppy
    end
    DoorPart.CFrame = FinishPos
end

function closeDoor()
    for i = 0, 1, Tick do 
        DoorPart.CFrame = DoorPart.CFrame:lerp(StartPos, i) 
        wait() 
    end
    DoorPart.CFrame = StartPos
end

Add a click detector in the door to fire the functions

local DoorPart = workspace.Door --(change your door part's name to 'Door')
local StartPart, FinishPart = workspace.StartPart, workspace.FinishPart

local StartPos, FinishPos = StartPart.CFrame, FinishPos.CFrame

local Tick = .1 --higher value will make the door transition look rough, really low values take too long. Suggest keeping this  greater than .1 and less than .3

function openDoor()
    for i = 0, 1, Tick do 
        DoorPart.CFrame = DoorPart.CFrame:lerp(FinishPos, i) --transitions the door part to the finish part's cframe
        wait() --don't add a number in wait() it makes things look choppy
    end
    DoorPart.CFrame = FinishPos
end

function closeDoor()
    for i = 0, 1, Tick do 
        DoorPart.CFrame = DoorPart.CFrame:lerp(StartPos, i) 
        wait() 
    end
    DoorPart.CFrame = StartPos
end

local IsOpen = false --determines if the door is open or not
local ClickDetector = Instance.new("ClickDetector", DoorPart)

ClickDetector.MouseClick:connect(function()
    if IsOpen == false then 
        print("opening door")
        openDoor()
        IsOpen = true 
    else
        print("closing door")
        closeDoor()
        IsOpen = false 
    end
end)