Make sure first that you are using the latest version of Traffic Travis by carefully following these instructions to download the latest version.
Be sure that there are no blank spaces BEFORE or AFTER the registration code when you enter it. Either will cause an invalid code to be detected.
The other thing you must check is that you are entering zeros as "0" and the letter o as "o" as this can also cause the error.
Please also make sure that your security software and router (if you have one) are not blocking Traffic Travis in any way, as these could also cause problems with the registration process.