Make sure you select the “Installable WordPress file only” option. If you select “All files & documentation”, you will not be able to install this zip as is. Instead unzip the file and then locate the installable theme file within it. Luckily most developers clearly label their files, so it should be pretty easy to find.
you can check this helpful article stylesheet error
Also you can unzip your purchased download (All files & documentation) 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.