I remember that it worked before I upgraded my wordpress. I also tried to enable it from Visual Composer Settings > Product License > Activate Visual Composer button, and it asked me to login to my envato account, post which it displayed following message:
NO LICENSES FOUND
It seems that you do not have any valid license for Visual Composer. What does it mean? It may be so that you got Visual Composer by purchasing WordPress theme which does not grant you with direct license for Visual Composer.