You need a rotating keycode to access the Bow in Banshee's workshop, but the original has been lost. You'll have to generate a new code following the same steps that Banshee did. First on Banshee's list is to get out into the field to seed a key generator with combat data. Seed data for a new keycode by completing Gambit matches or Vanguard strikes. Gambit Prime matches and Nightfall strikes grant the most efficient progress.