"Position is not a valid member of Model"?

hi guys

i have a game about transportation and now i am adding lifts. BUT MY SCRIPT DOESN'T WORK! it keeps erroring:

"Position is not a valid member of Model"

well, that's a stupid name for a part. but the problem is IT NORMALLY WORKS! the part is anchored and is ALWAYS there, but randomly when playing or testing that error occurs.

the part is inside a model and the model is targeted with an ObjectValue inside the script

main = script.Parent.Parent.Main
iM = script.Parent.isMoving
cM = script.Parent.canMove
iU = script.Parent.inUse
floors = script.Parent.Parent.Parent:GetChildren()  --where to look for floors
speed = 0       --current speed, changing it does nothing
maxSpeed = 4    --highest speed
tolerance = 5   --how many studs before destination the lift slows down
minSpeed = 0.4  --minimal speed when slowing down
acceleration = 0.2  --I hope I don't have to explain this?
hasStopped = false
holdPos = Vector3.new()
lastDirectionUp = false

wait(0.1)

function up()
    lastDirectionUp = true
    main.Anchored = false
    main.BodyVelocity.MaxForce = Vector3.new(0, math.huge, 0)
    main.BodyVelocity.Velocity = Vector3.new(0, speed, 0)
    iM.Value = true
end

function down()
    lastDirectionUp = false
    main.Anchored = false
    main.BodyVelocity.MaxForce = Vector3.new(0, math.huge, 0)
    main.BodyVelocity.Velocity = Vector3.new(0, -speed, 0)
    iM.Value = true
end

function stop()
    iM.Value = false
    main.BodyVelocity.Velocity = Vector3.new(0, 0, 0)
    if hasStopped == false then
        hasStopped = true
        if lastDirectionUp == true then
            holdPos = Vector3.new(
                main.Position.X, 
                (math.ceil(main.Position.Y*10))/10, 
                main.Position.Z)
        elseif lastDirectionUp == false then
            holdPos = Vector3.new(
                main.Position.X, 
                (math.floor(main.Position.Y*10))/10, 
                main.Position.Z)
        end
    end
    if (main.Position.Y +0.05) < holdPos.Y then
        up()
    elseif (main.Position.Y -0.05) > holdPos.Y then
        down()
    else
        main.BodyVelocity.Velocity = Vector3.new(0, 0, 0)
        speed = minSpeed
    end
end

function checkFloors()
    local canGo = true
    for i,floor in pairs(floors) do
        if floor:FindFirstChild("DoorClosed") then
            if floor.DoorClosed.Value == false then
                canGo = false
            end
        end
    end
    if canGo then script.Parent.canMove.Value = true
    else script.Parent.canMove.Value = false end
end

while wait(0.1) do
    if script.Parent.targetFloor.Value ~= nil then
        if (main.Position.Y -tolerance) > script.Parent.targetFloor.Value.Position.Position.Y
        or (main.Position.Y +tolerance) < script.Parent.targetFloor.Value.Position.Position.Y then
            speed = speed +acceleration
        else
            speed = speed -acceleration
        end
    end
    if speed < minSpeed then speed = minSpeed end
    if speed > maxSpeed then speed = maxSpeed end

    if cM.Value == true then
        if script.Parent.targetFloor.Value ~= nil 
        and (main.Position.Y +0.08) < script.Parent.targetFloor.Value.Position.Position.Y then
            up()
            hasStopped = false
        elseif script.Parent.targetFloor.Value ~= nil 
        and (main.Position.Y -0.08) > script.Parent.targetFloor.Value.Position.Position.Y then
            down()
            hasStopped = false
        else
            stop()
        end
    else
        stop()
    end

    r1 = Region3.new(
    Vector3.new(
        math.min(script.Parent.Parent.pos1.Position.X, script.Parent.Parent.pos2.Position.X),
        math.min(script.Parent.Parent.pos1.Position.Y, script.Parent.Parent.pos2.Position.Y),
        math.min(script.Parent.Parent.pos1.Position.Z, script.Parent.Parent.pos2.Position.Z)
    ),
    Vector3.new(
        math.max(script.Parent.Parent.pos1.Position.X, script.Parent.Parent.pos2.Position.X),
        math.max(script.Parent.Parent.pos1.Position.Y, script.Parent.Parent.pos2.Position.Y),
        math.max(script.Parent.Parent.pos1.Position.Z, script.Parent.Parent.pos2.Position.Z)
    )
)



plrFound = false

    for i,Part in pairs(game.Workspace:FindPartsInRegion3(r1,nil,math.huge)) do
        if Part.Name == "Head" then
            plrFound = true
        end
    end

    if plrFound == true then
        iU.Value = true
    else
        iU.Value = false
    end

    checkFloors()
end

sorry it's unorganized af lol (believe me, a lot of people have worse organized scripts)