The key part from the error is:
The theme is missing the style.css stylesheet.
WordPress docs on Main Stylesheet says:
In order for WordPress to recognize the set of theme template files as a valid theme, the style.css file needs to be located in the root directory of your theme, not a subdirectory.
I would suggest you to unzip the theme and look for
style.css. If the file is there open it; there should be some commented lines that will look similar to these:
Theme Name: Twenty Seventeen
Theme URI: https://wordpress.org/themes/twentyseventeen/
Author: the WordPress team
Author URI: https://wordpress.org/
Description: Twenty Seventeen brings your site to life with immersive featured images and subtle animations. With a focus on business sites, it features multiple sections on the front page as well as widgets, navigation and social menus, a logo, and more. Personalize its asymmetrical grid with a custom color scheme and showcase your multimedia content with post formats. Our default theme for 2017 works great in many languages, for any abilities, and on any device.
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Text Domain: twentyseventeen
Tags: one-column, two-columns, right-sidebar, flexible-header, accessibility-ready, custom-colors, custom-header, custom-menu, custom-logo, editor-style, featured-images, footer-widgets, post-formats, rtl-language-support, sticky-post, theme-options, threaded-comments, translation-ready
This theme, like WordPress, is licensed under the GPL.
Use it to make something cool, have fun, and share what you've learned with others.
The following fields should be in every main stylesheet:
License URI and
If any of the mentioned fields are missing I strongly recommend contacting the theme provider.
If you're the owner or the author of the theme make sure to include the mentioned lines and to include the
style.css file in the root directory of the theme before uploading the archived theme to your WordPress installation.
Also, make sure to read more on Main Stylesheets here: https://developer.wordpress.org/themes/basics/main-stylesheet-style-css/