This is a WordPress theme you have purchased here?
Opt 1: In your download page click download & select ‘installable WordPress File Only’ & use this (if you can’t see this then it is not a WP theme).
Opt 2: Are you sure you have the WP theme and NOT a HTML file?
Opt 3: Are you using your own hosting and domain NOT WordPress.com
Opt 4: Unzip the theme folder and use ftp to install it