"Misschien draag ik de rok."
Translation:Maybe I am wearing the skirt.
In a Dutch main clause, the verb always comes as the second word group. So if the subject is not at the very beginning, then it switches places with the verb. This is called inversion. For a longer explanation and some more examples, take a look here: https://www.duolingo.com/comment/3733010
It accepted "Maybe I'll wear the skirt". Sometimes you have to change the tense when translating. In English using the present tense in this sentence is awkward (do you not know what you're currently wearing?), but in some other languages, apparently including Dutch, it's a normal way to talk about the future if it's clear from the context.
Sometimes this works in English as well, e.g. "Tomorrow I'm leaving", but it doesn't work in this sentence.
You also can't go by what "this lesson" is about, since the sentence could appear without that context when using the "strengthen skills" feature.
Yeah, they certainly do that in German, so I went out on a limb and put "Maybe I'll wear the skirt" too. As you said Duo seems to think that's fine. Maybe it doesn't like the "will be wearing" part. (I'm not sure it's wrong but it seems like something that might slip through the cracks when they set up the correct answers.)