You may have accidentally declined the quest, I did that and I found it in the abandoned quest thingy near the vanguard

I was about to say check the kiosk and forget everything I said but then I saw someone already said it so ya.

Try talking to cayde but if that doesn't work the only logical answer is to move all your stuff to the vault put on white armor or armor you don't want replace your weapons with whites or greens and delete your character [spoiler]im not trying to mean this is the only way[/spoiler] and you'll get a good reason to use your spark of light make sure it's in your vault. I'm sorry you have to delete a character I really am. Oh and you'll need to beat the first level (cause you need a ship) you'll get to the tower. Go to vault use spark and go talk to Zavala to get your mission on Phobos. After you beat that report to cayde and do his quest. To get the nightstalker