This error happens when you try to install the main .zip file. This zip file also contains documentation, demo content…. and not only the theme so it can’t be installed.
You need to unzip the first zip and inside this zip, you will find the theme-name.zip file. This file is the correct theme file.
- Go to WP dashboard
- Go to Themes -> Upload New theme
- Select correct theme-name.zip file
- And at last, activate the theme