Hey, The only reason Xur wouldn't have the quest for you is if you already have one linked to your character. Since you don't have the quest in your log, it's probable you abandoned it at some point. You will need to complete that abandoned quest before Xur will give you another. Unfortunately, the quest is not appearing in the Quest Archive, so the only way to complete Xenology once abandoned is with another character. Create a new character, and use them to pick up and complete the quest. A week after collecting the cipher Xur should have the quest for your main again.