Because this is a fan-made hack and not an official game, it has issues:

The idea of playing classic Pokémon games—like FireRed , LeafGreen , Ruby , Sapphire , or Emerald —on an Android device is appealing to many nostalgic fans. Occasionally, fan-made ROM hacks (like Pokémon Indigo League , which may be based on the original anime story) circulate online. Before you search for a “Pokémon Indigo League GBA ROM download for Android,” it’s important to understand the legal and practical landscape.

: This is the software that "acts" like a GBA on your phone. You can find highly-rated options like John GBA or My Boy! on the Google Play Store .