Build the Palais Garnier is a French decision. Palais Garnier can be build by 
France if it has researched Expressionism and it gives 11 prestige.
There is no reason not to take this decision as soon as it is available.
The ingame text says:
The Palais Garnier was, from its inauguration in 1875 to 1989, the chief home of the Paris Opera. Designed by Charles Garnier in the Neo-Baroque style, the Opera House was widely regarded in its day as a masterpiece of modern architecture, and was widely imitated all over the western world. The Palais also served as the inspiration for Gaston Leroux' classic novel 'The Phantom of the Opera