You have to be logged in, so start with that. If you are not logged in, you will see a message at the bottom of the page, requesting you to log in.
And for some reason Google's chrome does not keep you logged in. So try Firefox, which does a better job at keeping you logged in.
Just tested it myself, but when logged in I was able to submit my answers.
I guess that it's due to a bug, that somehow an object or value was set wrongly for you. I do advise you to sent an email to firstname.lastname@example.org and ask them to reset your tests