I can't activate the license : Error code : System 12029
The 12029 error is usually an indication that something is blocking the connection to the CCleaner licensing server.
It could be a fault somewhere on the internet, in which case then as the message says try again later.
It could be your antivirus or firewall settings.
Try pausing/disabling your antivirus (and any other internet security apps you may be using) while you licence CCleaner, don't forget to un-pause/re-enable it afterwards.
If that works then you will probably want to whitelist (allow) CCleaner in your antivirus.
If you don't do that then CCleaner may not work properly as your AV may still be blocking it's connections.
If you are still having problems then email support at support@ccleaner.com