Extruding Parts of a NURBS Surface

I am attempting to extrude down along the Z axis the four vertices I have selected on the right side of the NURBS surface in this file:

This thread shows how to do the extrusion I am attempting:

I find I am unable to do so. When I attempt to extrude using the shortcut key sequence E Z, there is no extrusion. Instead it acts the same as the sequence G Z, and moves the vertices instead of extruding them. What is going wrong here?

My Blender version is 2.90.1

only way to extrude Nurb is to select end loop and extrude

you cannot extrude in the middle of surface

happy bl