This error happens when you try to install main .zip file. This zip file contains also documentation, demo content…. and not only the theme so can’t be installed.
You need to unzip first zip and inside this zip, you will find theme-name.zip file. This file is 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