“The package could not be installed. The theme is missing the style.css stylesheet.”
Assuming that this is a Wordpress theme
This error occurs when the file you are trying to upload into WordPress is not a valid theme file. Please go back to your downloads page and download the theme again, making sure to select “Installable WordPress file only.”
If you don’t see this option, then you may have purchased an item which is not compatible with WordPress. I would suggest contacting the author of the item you purchased to see if they offer a WordPress version and can help you exchange it.
Hope this helps!