The Monument wasn't unlocked, but awarded to old Clans that had already completed Dry Dock research before the RJ overhaul a few years back - cannot recall the specific update, but the (first) one that massively reduced construction time/resource costs in Rising Tide. As far as I'm aware, much like old Operation trophies, newer Clans simply cannot access the decoration.
Although, if making new ones, despite already having one is the issue... I'm afraid I've got nothing, sorry.