Pokémon Pokopia takes place in a post-apocalyptic version of the Kanto region. Humans have disappeared, the landscape has withered, and Ditto works to make the area habitable for Pokémon again.
The setting connects Pokopia to the original Pokémon world while presenting familiar geography as a reconstruction project. Ditto finds an old Pokédex and meets Professor Tangrowth, who investigates why people and Pokémon abandoned the land.
Kanto is sometimes mixed up with newer regions because Pokopia includes Pokémon from Generations I through IX. The broad species roster does not change the story location: the devastated region is explicitly Kanto.