Hey,
If Xur does not have the quest for you, then it's likely you've picked up the quest and abandoned it.
Unfortunately, the quest is currently not appearing in the Quest Archive, so the only way to complete it once abandoned is with a different character.
If you are unable to pick up the quest with any of your characters, then your best option is to wait for Bungie to change the quest so that it can be retrieved from the kiosk.
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So we have to wait, delete our character or start a new character?
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If you've abandoned Xenology, then the only known workaround to obtaining it again is still to complete the quest with a different character. Since you only have one, you can create a new one and use them to pick up the quest, complete it and collect the cipher. After that, Xur should have the quest again for your main the next time he appears.
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That really blows - thank you for your response!