Where is the galactic building in eterna city Pokemon diamond to get bike?

There is no "galactic building" in Eterna City in Pokémon Diamond.

To get the bike in Pokémon Diamond, you need to go to Cycling Road which is located south of Eterna City. You'll meet a man who will give you the Bicycle after you show him the Pokémon Trainer's License you obtained in Jubilife City.