The code samples documented on the Instance:Clone function page attempts to index a non-existent variable due to a slight oversight.
The issue lies on line 4.
local copy = model:Clone()
Error
should be
local copy = original:Clone()
Code sample w/ fix
-- Get a reference to an existing object
local original = workspace.Model
-- Create the model copy
local copy = original:Clone()
-- Parent the copy to the same parent as the original
copy.Parent = original.Parent
-- Move the copy so it's not overlapping the original
copy:SetPrimaryPartCFrame(CFrame.new(0, 50, 0))