There's a lot of prerequisites for the assembly option to show up. Here is the update notes "Werewolf event finished, Orc event finished, Lizard invited to the fair, Tao invited to the fair, NC guards invited to the fair".
The ingame notebook mentions that you need to invite Tao to Bareshade, but doesn't specify that you need to also invite him to the fair. That's what stumped me for a bit. The other thing might be that you haven't invited the NC guards yet, which requires you to give Everett a pastry sweets. It's a whole other chain.