Hey, Xur will not have another quest for you until you complete the one you abandoned. Unfortunately, the quest is currently not appearing in the Quest Archive, so the only way to complete it is with another character. You'll have to create a new character, and have them pick up and complete the quest. A week after collecting the cipher, your main should have access to the quest again. If you have a complete quest in your inventory, and Xur does not give you the option to collect the cipher, then you likely already have a cipher in your inventory. Spend the cipher, and speak to Xur again to complete the quest.