Do not upload the Themeforest download package directly to your Wordpress site, because this will lead to errors. Make sure to Unzip this package and inside you will find the theme zip archive: ella.zip. This is what you will need to upload to your Wordpress installation
If you upload / Install the Ella theme and you get one of the following errors, then you have forgotten to unzip the download package.
This theme is invalid
This theme is missing a style.css