Rainbow Cloud is a legendary Pokémon, not a location.
To encounter Rainbow Cloud, you need to fulfill certain conditions in New Pokémon Snap:
* Complete the Photodex: You need to photograph all the Pokémon available in the game.
* Reach a high enough score: You need to achieve a certain score on each course.
* Obtain a specific item: You'll likely need to find and use a specific item to trigger the encounter.
The exact requirements are still being figured out by players, so keep an eye out for online guides and walkthroughs.