The official Pokémon website has announced that Pokémon Rumble Rush service had ended and can no longer be played as of July 22nd, 2020. The game was announced on May 15th, 2019 and was released worldwide on May 22nd, 2019 on Android and July 23rd, 2019 on iOS.

Pokémon Rumble Rush Service Ending

The mobile game is no longer available and game service has ended.

Pokémon Rumble Rush has been retired and services for the mobile game have ended. From this point on, you cannot install it for the first time or update this app. You also cannot play any previously installed version of the app. Additionally, unused Poké Diamonds can no longer be used or redeemed.

Thank you for enjoying Pokémon Rumble Rush.

Source: https://www.pokemon.com/us/pokemon-news/pokemon-rumble-rush-service-ending/