How to get a players torso from its player?

Wait for the player's character to load using the CharacterAdded event, then use WaitForChild() to grab the HumanoidRootPart of the character, and then set its CFrame to where you want to teleport them to.

local DefinitelyPlane = workspace:FindFirstChild("1")
local PlaneName = 1
local Plane = workspace:FindFirstChild(PlaneName)
local PositionXX = 87.18
local PlayerQuantity = 0
local Full = false

if script.Cars >= 10 then
    Full = true
end

game.Players.PlayerAdded:Connect(function(Player)
	PlayerQuantity = PlayerQuantity + 1
	if script.Cars < 10 then 
		local RPlane = Plane:Clone() 
		script.Cars = script.Cars + 1
		PlaneName = PlaneName + 1
		PositionXX = PositionXX - 15
		RPlane.Parent = workspace
		RPlane.Name = PlaneName
		RPlane:MoveTo(Vector3.new(PositionXX, 5.1, -922.8))
		print("Not Enough Players")
		local char = Player.Character or Player.CharacterAdded:Wait()
		local hrp = char:WaitForChild("HumanoidRootPart")
		hrp.CFrame = CFrame.new(x,y,z) --replace with the coords or the position of where you want to teleport the player
	end
end)