You have practiced all of your words. Try doing some lessons or translations.
In six months of duolingo I've never had this message appear, but today it's preventing me from doing my usual daily practice.
Has something changed recently?
Actually, so far as I know, Duolingo never requires that you do translations. Maybe they will in the future, but, for the time being at least, they never do. At least that is the impression I've gotten from reading some of the past forum posts made by Duolingo staff. Maybe one of them will chime in here and give the definitive answer.