The error is par.Size.XScale, yet I don't know what other alternative there should for to fill that spot. The error says "XScale is not a valid member."
par = script.Parent
script.Parent.Visible = true
print("Act. Completed. Activating movement.")
repeat
wait(0.1)
par.Size = UDim2.new(par.Size.XScale + 0.05, 0,0.005,0)
until par.Size.XScale == 0.5

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Of course, I am not much of a Gui developer. No wonder I couldn't see the issue.