Object Not A Valid Part Of StarterGui?

Whenever I run this piece of code (by clicking on a button), it gives me the error code, "ProgressBar is not a valid part of Frame".

ProgressBar = script.Parent.Parent.Progress

					ProgressBar:TweenSize(UDim2.new(1, 0, 0, 12), "Out", "Quad", 2)
					while Percent < 4 do
						print(Percent)
						Percent = Percent + 1
						wait(.5)
					end
					if Percent == 4 then
					ProgressBar.Size = UDim2.new(0, 0, 0, 12)

There is more code, but I don't think it's related, so I didn't post it. K thanks.