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).

plane is not a valid member of seat means Plane does not exist in the Seat