Why am I getting this error?
This is a WordPress error message that is misleading and it actually means that either the wrong file is being uploaded or the theme files packaged differently than expected as some authors may have zipped it for you, others do not.
Most commonly, it happens because of accidentally clicking "Download All" rather than downloading just the "Main File" which is used for installing.
How do I download the correct files?
First, you will navigate to your purchases tab.
You will then click the download arrow to the far right, then you will want to download two files. One is the "Help Files" these will give you step by step instructions on how to install your theme. The second will be the "Main Files" these are what you use to upload to your WordPress site.
What if I am using a Mac?
You may need to compress the theme files. This file is often called the name of the theme within the zip you have downloaded (i.e. the-core-multi-purpose-wordpress-theme-theme-files). Right click this file and choose to compress, then upload only that file.