From Bulbapedia, the community-driven Pokémon encyclopedia.
(Japanese: マナーロ・スタジアム Manalo Stadium) is an anime-exclusive very large floating structure in the Alola region found near Aether Paradise.
It was built by the Aether Foundation to host Alola's first League Conference. It first appeared in The Dealer of Destruction! while it was still in construction, when Professor Kukui announced the Alola League. The construction was completed in SM127.
It reappeared in SM128 as some of the entrants for the Manalo Conference signed up and explored some of the stadiums sights. The Conference itself began in the next episode.
Besides the battlefield, the stadium also houses a Pokémon Center, a shopping mall, and multiple restaurants and food trucks.
Manalo Stadium under construction
Manalo Stadium's battlefield