Why am I getting this nil value error?

I am trying to make a script that picks a random model, clones it, and changes the parent so that it isn't picked again, however, I am receiving an error that I do not know how to fix:

20:00:42.317 - Players.Player.PlayerGui.ScreenGui.Buy New Floor.TextButton:3: attempt to index a nil value 20:00:42.317 - Stack Begin 20:00:42.318 - Script 'Players.Player.PlayerGui.ScreenGui.Buy New Floor.TextButton', Line 3 20:00:42.318 - Stack End

Here is my code (Normal Script, not local. Also, UnbuiltFloors and BuildFloors are created in another script that is fully functional. They are models.):

reception = game.Workspace.Lobby.Spawn

local list = script.Parent.Parent.Parent.Parent.Parent:FindFirstChild("UnbuiltFloors"):GetChildren()
local m = list[math.random(#list)]

script.Parent.MouseButton1Click:connect(function()
	
	
print(m)
print(#list)
	
	list = script.Parent.Parent.Parent.Parent.Parent.UnbuiltFloors:GetChildren()
	m = list[math.random(#list)]	
	
	if m.Name ~= "" then
		local room = game.ServerStorage:FindFirstChild(m.Name):Clone()
		room.Parent = workspace
		room:MoveTo(reception.Position)
		
		game.ServerStorage:FindFirstChild(m.Name).Parent = script.Parent.Parent.Parent.Parent.Parent.BuiltFloors
		
	end

			
	
end)

FindFirstChild will return nil if the model is not found. Try using WaitForChild in place of all the instances where FindFirstChild is found in the script.