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.
The reasons is that you are trying to install the wrong file. Probably you are tying to install the main download folder from ThemeForest named "themeforest-13373220-uncode-creative-multiple-wordpres-theme.zip".
The theme is inside this folder and it's named "uncode.zip".