I finished all quests, the co-op event, the una's daily quest more than five times (it's six today) and got the achievements, but still this quest doesn't show up. I have no idea.
In case you can't start this quest even though you think you have fulfilled all the conditions:
it might be because you completed the Green Co-op quest on a different character. The quest doesn't show on the alt where I did the Unas with but on my main it does. Achievements and Unas are Roster-wide anyway but apparently co-op's arent.
Adding to this, make sure you've completed all the conditions and required quests as some of them can be easily missed. In my case since I was lazy, I used Una's completion pass for the super secret materials daily and that caused me to not have the hard candies achievements (take pictures of elements) Hope this helped, gl in your honing,card packs openings and raids
Visit the quest's page. There you'll see a link to the NPC/Object, which starts the quest. Click it and you'll see the map with location of this NPC/Object.