How do i fix "attempt to call method 'clone' (a nil value)" in the script?

When I open the "Output" in the View tab. I shows me

"Players.Player1.PlayerGui..:Insert/CleanGui:..InsertFrame.CleanWorkspace.Script:2: attempt to call method 'clone' (a nil value)" in red

I want it to regenerate models in a folder but I don't know why it doesn't work. The folder is called Noobs and the models in the folder is called NPC.

model = game.Workspace.Noobs:GetChildren()
backup = model:clone()

function regenerate()                                                                        
    model:remove()                                    
    wait(2)                                                               

    model = backup:clone()
    model.Parent = game.Workspace                                                                                   
    model:makeJoints()
end


function onClick()
regenerate()
end

script.Parent.MouseButton1Click:Connect(onClick)

Thanks

I am new at scripting