Hi
First thing please confirm that you have purchased WordPress theme or not. If you purchased WordPress theme and can’t install then please check the following:
You have to download the Item from your Envato Dashboard Downloads then have to unzip and you will see a .zip file name with matching your theme name. You have to install that themename .zip file.
you can check this helpful article stylesheet error
Also you can unzip your purchased download and check the theme documentation hopefully you will get necessary support in documentation how to install and all necessary help. If you have still problem with the theme then The best way to get the support is Contact your purchased Theme Author.
Contact Author because Item Author are best expert about their Item functionality.
Thanks