Point to note: Depending on your purchased theme, screenshots can vary.
If you have received this error then there is a easy fix available for the issue. It's actually not an issue.
You may have received this error because you have uploaded whole theme package and not the correct theme file.
Please go to https://themeforest.net/downloads
and for your purchased theme, click on Installable WordPress file only link to download installable theme file.
You probably have downloaded complete package from All Files & documentation option. Even if you have that file, you can unzip the file and within unzipped folder you'll find theme file. Unzipped folder should look like below screenshot.
In above case, Qube.zip is the theme file which you need to upload. This is just an example. Depending on your purchased theme, themename.zip should be selected to upload.
This should fix the issue and you can install & activate the theme. If this still doesn't work, please feel free to open a support request with temporary access of your WordPress installation and one of our support agents will check it as soon as possible.