Stack resolving with planeswalker and instants
Nissa, Voice of Zendikar is a Planeswalker, which means that you can only activate her abilities on your turn, and Devour in Flames is a Sorcery, which means your opponent can only cast it on their turn. In other words, if your opponent is able to cast Devour in Flames, then you don't have a chance to activate Nissa's ability.