If you are getting the error similar to this in WordPress when you upload the theme to install it:

Then you are probably uploading the zip file you downloaded from the ThemeForest directly. In order to install the theme properly, you have to first unzip the file you downloaded.

Inside unzipped file you will find another zip file, which has the same name as the theme you purchased, for example if you purchased Hairpress, the zip for upload will be named hairpress.zip, for Carpress it will be called carpress.zip and so on … You should upload that file.