goldo00, there are two typos in BKitems.rpy which block you from building weapon racks. Both "Weapon rack" and "Weapon rack XL" are marked as "Utilities" instead of "Utility", and because of that they're not picked up by the carpenter screen.
To fix a new game, edit lines 387-388 of BKitems.rpy and change "type='Utilities'" to "type='Utility'".
To fix an existing game, enter the console and type:
Python:
furniture_dict['Weapon rack'].type = 'Utility'
furniture_dict['Weapon rack XL'].type = 'Utility'
A nice side effect is, you can now actually manage to make the carpenter icon disappear if you build everything