Hey there. If you can’t get one from Xur and haven’t already gotten one this week, then most likely you had the quest in your inventory at one point and deleted it. The current workaround is to use a different character that hasn’t deleted to quest to pick it up. If all your characters have deleted the xenology quest at one point or another, you’ll have to wait until Bungie issues an official workaround. As an aside: if you are able to pick up the quest, make sure you don’t have a cipher in your inventory or vault; make sure you also have space for it.