My capitol leveled up to level 5, and I selected the Onyx Thone. On the window where the choice of buildings was presented, the icon for the Onyx Throne was blank - other choices were not. In the build list for the city, the Onyx Throne is shown, and the icon there looks like a throne, but when trying to place the building, it can't be done.
Saved game is here. City with unbuildable Onyx Thone is Imperium.
