Unlike other VC systems, GBA injection requires a gba_bios.bin file to work properly. NSUI will prompt you to provide one. You can dump this from an original DS or find it online (legally complex).
By following this guide, you can now:
is the process of taking that official Nintendo "wrapper" (the emulator) and replacing the game data inside it with a different game. 3ds virtual console injection
⚠️ : Installing custom firmware and using injected ROMs of games you do not own exists in a legal gray area. This guide is for educational purposes and assumes you own original copies of any games you inject. Unlike other VC systems, GBA injection requires a gba_bios