Would 'obligatory' fit as well as an alternative meaning?
That's an accepted translation.
Hmmm... Do you think Duolingo sometimes adds complication for the sake of added complication?
The preferred translation is usually the most common one. However, translations that look too much alike are omitted in this course, as it would make things too easy ;)
In this case, however, 'mandatory' is indeed the most common translation.
Aha! Just as I thought!