Translation:No one heard the pilot's announcement.
In French, we often invert the ne and the closer (jamais, personne, rien, aucun.e) when the negativity is in the subject, not in the verb.
I see no one. — Je ne vois personne.
No one sees me. — Personne ne me voit.
There is nothing that can separate us. — Il n'y a rien qui peut nous séparer.
Nothing can separate us. — Rien ne peut nous séparer.
Likewise, here, the subject is "no one", and is negative. So personne ne is used, not ne personne.
One of the problems with Duolingo is that there is very little "teaching". It gives us exercises to do and it give "tips" but does NOT do a good job of explaining things. All of the explaining or teaching appears to take place in discussions. This is a serious flaw, from my standpoint.