In English, it makes no difference whether you say "the first," or just "first," so why does Duo mark it wrong if you leave out "the?"
In english there is a difference
Maybe because in portuguese you can also leave the "o" away.
But i guess it should be accepted
He is number one - should work as well. I'll report.
"Number one" does not always mean the same as "first," in English, though. In fact, I would usually interpret your sentence, "He is number one" as "He is the best."