You have to use a mysterious candy now to evolve your level 100 Riolu. Make sure its daytime as navneeth mentioned.
You can purchase mysterious candies in all four department stores (Celadon, Goldenrod, Lilycove, Veilstone) for 5k pokedollars, for 2 PvE coins at PvE Coin Masters or for 1 PvP coin at PvP Coin Masters.