What does plane is not a valid member of seat mean?

I'm making a tycoon and whenever I put the plane into the tycoon, the tool to fly the plane doesn't pop up anymore. I get the words (plane is not a valid member of seat) in the output but im not sure what this means. It brings me to the second line of the code below. Could anyone help me with this?

while (not script.Parent.Parent.Parent.Cloned.Value) do wait() end
local tool = script.Parent.Plane:clone()
script.Parent.Plane:remove()    -- If not removed, the player will have 2 of the tools, one being like a "Ghost." This glitch seen on RO-Port Tycoon, and this line fixes it

script.Parent.ChildAdded:connect(function(obj)
    if (script.Parent.Parent.Parent.Dead.Value) then return end
    if ((not obj:IsA("Weld")) or (obj.Name ~= "SeatWeld")) then return end
    local p = game.Players:GetPlayerFromCharacter(obj.Part1.Parent)
    if (not p) then return end
    if (not p:findFirstChild("Backpack")) then Instance.new("Backpack",p) end
    if (p.Backpack:findFirstChild(tool.Name)) then return end
    local t = tool:clone()
    t.Plane.Value = script.Parent.Parent.Parent
    t.Parent = p.Backpack
    script.Parent.Parent.Parent.Origin.Value = script.Parent.Parent.Parent.Parent
    script.Parent.Parent.Parent.Parent = p.Character
end)

Workspace.tycoonkit.tycoons.blueteam.purchases.plane.planekit.plane.mainpart.mainseat. handling (with plane tool on the same level as handling).