One of the most common reasons people have issues is because they try uploading the Full Theme Package zip that includes sample data, documentati, etc which displays the “The package could not be installed. The theme is missing the style.css stylesheet.” error. Please make sure you are uploading only the Installable WordPress file.
When you download the theme from ThemeForest it will give you 2 choices: one is to download the Full Theme Package which you will need to extract to locate the theme and the other one is just the Installable WordPress file. Please pay close attention to this.