The files that you downloaded from Themeforest contains amongst the theme files also help files, demo content files, Photoshop files and more.
When you install this file that you downloaded from Themeforest the WordPress installer sometimes struggle to see the theme and all the required files that’s necessary for a theme to function amongst all the files , and then this message is returned.
The best way o install the theme is to unzip the files that you downloaded from Themeforest and install the zip file in the folder. The name of the zip file will usually be the same as the name of the theme.
This would clear up the message, and the theme will be installed without a hitch.