What is wrong is that you have a thin-walled solid body that you meshed with solid elements.
The best way to mesh this body is to open the geometry in SpaceClaim and use the Midsurface feature to make surfaces at the center of the thickness of the solids. Then in Mechanical, it is easy to mesh the surfaces. The shell elements are assigned the thickness of 20 mm.
You didn't say what version of ANSYS you are using. Attached is a 2019 R3 archive.