I am glad to see this thread, as I am having the exact same issues, and know that I am not alone or have self-created a problem. I have two cruises booked and cannot get either to show up. I have had the underage error pop up, as well as an error saying that the cruise does not leave within 20 days, which is true, but irrelevant. I have also had an error saying that the registration failed - call my travel agent, which happens to be MSC.
My first cruise does not leave until 11/12, so I will wait to see if it resolves. I had completed registration. We shall see what happens