Edit 2016/09/23 : this answer can be a little outdated. If this doesn't work, you can explore @ashish answer just bellow.
During the Sonata User Bundle setup, you are advised to map doctrine:orm to ApplicationSonataUserBundle, and this cannot be done during sonata:easy-extends:generate SonataUserBundle because the ApplicationSonataUserBundle doesn't exists yet.
So before lauching your sonata:easy-extends:generate SonataUserBundle command, you can comment the ApplicationSonataUserBundle line :
# app/config/config.yml
[...]
# Doctrine Configuration
doctrine:
  [...]
  orm:
    auto_generate_proxy_classes: %kernel.debug%
      entity_managers:
        default:
          mappings:
            # ApplicationSonataUserBundle: ~
            SonataUserBundle: ~
Then you launch your sonata:easy-extends:generate SonataUserBundle command,
And finally, you remove comments (the #) on ***ApplicationSonataUserBundle: ~ **
Bundle “ApplicationSonataUserBundle” does not exist or it is not enabled error is gone.
Have a good day.