I just claimed my exotic cipher from the last xenology quest and used it for an exotic engram. I don’t have and exotic cipher in my inventory yet I can’t access the Xenology quest from Xur. The option to go the left of Xur’s UI isn’t there.
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Hey, Xenology can only be completed once per week per account. If you collected the cipher from a previous week's quest today, then that counted as your completion. You will need to wait until Xur returns next week to be able to pick up the quest again.