Why doesnt this work?

You cannot set a CFrame value to a Vector3 value. To fix this you would write it like this:

local Guy = game.Workspace.Guy --Going to assume Guy is actually in Workspace
Guy.CFrame = Door.CFrame
--or
Guy.CFrame = CFrame.new(Door.Position.x, Door.Position.y, Door.Position.z)
--or
Guy.Position = Door.Position -- would appear above the door because of how position 

--Okay so Guy is a apparently a model, to move a model you would do this:
Guy:MoveTo(Door.Position)