A common issue that can occur when installing a WordPress themes is “The package could not be installed. The theme is missing the style.css stylesheet.” error message being displayed when uploading or activating the theme.
One of the reasons you may receive this error is if you have uploaded the incorrect file. You have 2 options for downloading your theme from ThemeForest.
1. Installable WordPress file only. (below)
2. All files and documentation (full zip folder). You will need to extract and locate the installable WordPress file. (below)
Maybe you need the article: Installing SEA via WordPress