If after clicking “Install Now” you see an error that “The package could not be installed. The theme is missing the style.css stylesheet,” then you probably didn’t follow our instructions from step 1. You more than likely downloaded the “All files & Documentation” instead of the the installable theme. Go back and double check that you’ve downloaded the correct file.
For more details about themeforest WP installation
Definitely Author should to reply/assist you. But I would like to post a comments in theme comments page and let the author know that you have opened a support ticket: