When you receive a Server Busy message, it usually means that the server is under maintenance.
Head over to the Brave Frontier Global Facebook page to check for the server maintenance schedule before trying to play.
Refrain from forcing your device or game account in connecting into the game during server maintenance to avoid account problems in your side.
However, if the error message persists you can do the troubleshooting method found on this article here.
If that does not solve the issue contact our Game Support.