We are required by U.S. law to ensure that only people of legal age can register for an JibJab account. It's likely that you chose the incorrect birth year when signing up, and that's why you are being told you are ineligible. 

Unfortunately, and also in accordance law, once this happens, you will be unable to register for 24 hours while using the same internet browser.  However, once you have waited this 24 hours, you will once again be able to register with the same internet browser, but please ensure that you put in your correct birth year.