Hey there. What you have there is the incomplete 'quest' version of the weapon. You can see this because it only has a single shot in the magazine, and it is missing the usual weapon perks. When you first obtain the weapon, there were additional steps to take to complete it - kill Cabal with it, and complete more Leviathan encounters to collect emperor's seals. That would complete the weapon, adding the second shot to the magazine, and adding various perks. It would also then unlock in collections, and show up on collection badges, and it is this version that has the catalyst slot. As The Leviathan has now been vaulted, along with the last steps of the quest, it is no longer possible to complete the quest to get the complete version of the weapon. Instead you will have to purchase it from the memorial kiosk at the Tower.