Missing the style.css stylesheet error
A common issue that can occur when installing a WordPress theme 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.
Lots of customers and WordPress users are complaining an useless theme pack while they're trying to install it on your self host WordPress. It sounds a serious trouble, but actually it doesn't. When you buy a premium theme, there always are lots of resources come along with the theme - and authors basically put them all into a pack.
WordPress core always checks for style.css file inside new uploaded theme pack to get information about the theme. But new users - normally - will bypass the instruction guide and upload the pack directly onto their WordPress. The system obviously can't recognize the pack is a theme pack because, inside the pack you've just uploaded there is no style.css file - even in child folders.
Okay, let's focus on the problem. The package you've downloaded from the marketplace often has parent theme, child theme, sample data, and documentation. Just make sure you are uploading only the installable theme (that inside the package you download which is called theme-name.zip) through your WordPress dashboard. Then I believe everything will be fine.