Usually the problem is that your exercise was "check all the correct answers" and that means that there may be more than 1 correct answer. You have to check both "Jem" and "Ja jem" at the same time.
in Polish ( as in Russian) pronouns are often omitted. but this is not a bug
Тут есть два 'самых правильных' варианта, нужно отметить оба: "Jem" и "Ja jem".
Marek, my respect! I only understood the meaning of the comment, after I read your answer. And I'm a native speaker!