I was playing and got the notification that I had finally finished doing the Witherhoard Catalyst questline, so I went to Banshee-44.
But when I try to claim it, this happens. I cannot equip it and says I have to do the mission. However, the game recognizes I did the mission already because when I verify the "abandoned quests", it is not there.
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You did the quest to get the catalyst, now you need to get kills with the weapon to unlock the catalyst