The 13:6 error indicates a problem with the DRM (Digital Rights Management) / unlocking process.
This error typically occurs when the computer is missing a driver which enables the unlock code to be verified. To resolve this, please go to the following site: http://www.tagesprotection.com/main.htm
Then click on "Drivers". You can update to the latest version; be sure to select 64bit or 32bit, according to your OS.
Afterwards, be sure that you are connected to the Internet, and please try unlocking the game again.