You are trying to install whole theme package (downloaded zip file).
But you need to unzip downloaded package first and install just theme file; e.g. floris.zip
If you still have issues, please try install the theme through FTP:
Install Via FTP
Your second installation option is to upload the theme via FTP.
For this method first log into your site via FTP and browse to your folder located at wp-content/themes.
Unzip the theme file you downloaded from Themeforest and upload only the extracted Floris folder to your server.
If that still not solves your issues, please feel free to create a ticket for us and we will do help you with your Installation