Many of the Beast Tribe quests require you to go complete FATEs. The quests will display where that FATE will appear... but it's will appear, just like any other possible FATE. You just have to wait for it to show up; maybe clear some other nearby FATEs to make the game cycle them a bit faster.

That particular one requires you to defeat a mini-boss in a FATE, which is a bit tricky to do by yourself unfortunately.