It should also be accepted now, but unfortunately it takes some time for the changes we make in the Incubator to be active for users (sometimes as long as two weeks).
Still, please report (with the button in the lesson, not in the discussion) if it's not accepted, it's still possible something got missed!
It can go either way, although depending on which way you put it you're placing the emphasis in a different spot.
In Latin all personal pronouns can be omitted. You can say "ego sum vir" or "ego vir sum" (the verb usually comes last) but it is not necessary and puts some emphasis on the "I"
Obviously this sentence uses a non-omissible name and not a pronoun, but the rest of it applies equally.
"Marcus puer est" is the default order. Other orders are equally grammatical, but not semantically interchangeable because they serve to emphasize one thing over another.