They say they recommend
- A device with at least 2GB of RAM.
- Android OS 7.0 or higher.
- Android OS 5.0 or above / 64
they also say
- We do not guarantee functionality on all devices listed above.
- There may be cases where the app does not function properly due to your device’s capabilities, specifications, or particular conditions for using apps.
- It may take time to become compatible with the latest OS.
Though I don’t think these recommendations and notes have anything to do with it. It should appear on Google Play.
Also, the game is free to start, so you can't pre-order, but you can pre-register, not sure what that does though besides notify you when the game is released.
Source: Pokémon Masters (Google Play)