I have 2 sets of points at different height, one at a higher elevation with the other at the lower elevation. I'm trying to extrapolate lines for each point at the lower level to the points at the higher level. How do I do it?

I had a quick look at your file.
Have you thought about how you would like these points connected? With and add sop you can create new lines: add sop->polygons->by group-> skip every nth point (I set the number of points to around 140 which is equivalent to the amount of points in one of the inputs.)

That results in a mess of lines connecting the two meshes, I doubt that is what you want.

Would you want only the closest points to connect to each other? Generally with operations like this it is easier if you have the same point count on both meshes. (You could snap one of the meshes to the other to maintain the point count but change the shape.)

If you perhaps give a bit more info on what you are ultimately trying to do, this might help as well.