Petra is holding it hostage until you complete all of the Reef quests.
This includes The Taken War: Petra, Lost and Found, and The Wolves of Mars.
She also has to like you enough (Queen's Wrath Rank 3).
Some have reported after completing all tasks they need to wait until the next daily reset.
English
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I didnt have to wait for reset Got the gun when turning in the last quest
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So did I, but some people have reported that it happened to them.