Here's why and what you can do:
* No$GBA doesn't have built-in online multiplayer: No$GBA is an emulator, not a console. It doesn't have the hardware or network capabilities to connect directly to a real Nintendo Wi-Fi network or other players.
* Nintendo Wi-Fi Connection is discontinued: The official Nintendo Wi-Fi Connection service was shut down in 2014. Even if you could connect, it would be useless.
Alternatives for online play:
1. Use a dedicated online emulator service: Some online services like RetroArch with its online features, can provide a way to play against others online using emulators. These services often use different network protocols than the original Nintendo Wi-Fi Connection.
2. Find a local friend with a compatible console: If you want to play with someone close to you, try connecting two compatible consoles to the same local Wi-Fi network.
Important Note: Playing Nintendo games online using emulators is a grey area legally. Some games may require a paid online subscription service. Always respect the rights of developers and publishers when playing games.