Too many nodes. You may have gone too far when creating the polyline. Use "Close" before you get to the end. You had the Start and Finish in the same place (as it should be), but there was also a third node in the same place that caused the hiccup. Once that was moved (deleted), the 3D object filled in.
[attachment deleted by admin]
Designer, Deluxe, (Professional, Expert, Basic), Platinum
Windows 10 Pro (1903), 64-bit