Are you actually unzipping your download package first on your desktop, as specifically described here (even giving the example of the error you’ll receive if not doing it properly):
https://startupwp.com/topic/installing-your-theme/
In order to get to startuppro.zip – the file you need to install, not the whole package that you downloaded.
No one else has ever had this problem, there might be something wrong with your WP install. If for some reason, you’re still unable to install, would you let us contact you by email to get your login and install the theme for you?