Pokémon Pokopia takes place in a post-apocalyptic version of Kanto.
Kanto is the setting of the original Pokémon games and is familiar from Pokémon Red, Blue, Yellow, and their remakes. In Pokopia, however, the region is not presented as a conventional journey through towns, routes, and gyms. It has been abandoned by humans and has become dangerous and inhospitable to Pokémon.
Ditto awakens in this altered world and begins restoring habitats so Pokémon populations can recover. The player gradually transforms barren spaces into functioning environments by planting, building, crafting, and attracting Pokémon with suitable habitat features.
The setting can be confused with other Pokémon regions because the game includes Pokémon from Generations I through IX. Those species do not change the story’s location: the world being rebuilt is specifically a future or post-apocalyptic version of Kanto.