If you have a valid purchase code then this error means for some reason your server is unable to reach hostingbilling.net
You can get around this error by changing the server URL the installer is trying to reach
To change the URL please follow these steps
In the files that you uploaded open application/config/
Edit the file named constants.php
In line number 30, change this:
This should allow you to continue and if it doesn't please send an email to email@example.com for quick assistance.