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)