Talk to one of the two guards inside the hut in Indigo Plateau to battle the Elite Four which then moves on to Champion Gary. The PokéCenter is unavailable during your battle with the Elite Four until you defeat all members or are defeated yourself.
Snow Canyon was built for the 2016 Christmas Event and is still open to players. Indigo Plateau is entrance to the area, but you must complete a series of missions in order to pass the invisible barricade. This is now free for anyone who has 8 badges.