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
gui.XScale isn't a valid property, to get the X scale it's just
par.Size.X.Scale
If you want the offset
par.Size.X.Offset