As of October 31, 2019, at 01:59 UTC, services for Pokémon Duel have ended. You are no longer able to install or update the game or transfer your account. Additionally, if you already have the game installed, you will not be able to continue to use the app except for certain functions. The developers have implemented a function that allows you to view your Library when you open the app now that the service has ended. Library data that has been downloaded ahead of time is needed for the Library function. The Library function does not allow you to view things like the figures you had, their experience level, and fusion-based upgrading. It’s possible that you will become unable to use the Library function due to OS version upgrades, device change, or other issues—regardless, the developers are unable to respond to such issues now that service has ended.
Source: Pokemon.com
Pokémon Blog 