Well, you've spelled "height" wrong, but that's not really your error.
A ConvexPolygonShape3D does not have a
As a beginner, I highly recommend against using the Godot 4 alpha. There are still bugs, and there is very little documentation because it is not a release version yet. You would be much better off sticking to 3.x which is well documented and has plenty of examples for you to learn from.
You can move to Godot 4 when it is released as a stable version.