But this highlights an annoying aspect of Duo. If you use the wrong form of "tout," as I did, it's not accepted for that reason. That's fine. But then you're led to conclude that the verb was what you got wrong. And that's confusing.
I think it's always just "tout" when you mean "everything". "tous", "toute" and "toutes" come into play when you are specifically talking about a set of things, eg: All the cyclists, all the teams, all my good ideas