Since Super Mario Flashback is a fan project, it is not available on the official Google Play Store. You will need to download the (Android Package Kit) from a trusted third-party source. Step-by-Step Installation Guide:

The game is currently on hiatus with no set release date.

Super Mario Flashback combines the core mechanics of traditional 2D Super Mario games with elements introduced in the series’ 3D entries, such as Mario’s full moveset and a health system. While the feel of the game takes some getting used to, everything else in Flashback feels true to a Mario game, right down to the charming animations.

If you are looking for a verified, secure mobile experience, you can check out Nintendo's official mobile platformer, Super Mario Run on Google Play . Share public link

Super Mario Flashback APK Android: The Ultimate Fan-Made Platformer Guide