@Vijay Bhardwaj, @Raisa Melo Lima: Ray-tracing is disabled in the free (education or evaluation) copy of PyMOL obtained from Schroedinger. You can produce a high-resolution image using the png command, but without ray-tracing.
As I mentioned above, to obtain a free, fully functional version you have to
use the open-source version of PyMOL from the sources linked in my previous post.