Please try the following steps to see if any of them resolves the problem:

• Please make sure that your device meets the system requirements.  
  If your device does not meet the system requirements, the application may not start.
• Restart the application.
  Please close the running application and try restarting the game.
• Restart your smartphone.
  Turn off the device and then turn it back on.
• Switch to a stable connection or switch from Wi-Fi to a carrier connection.
  If you are using a carrier line, please make sure that your connection speed is not limited.
• Set [Graphics] to lightweight *When the game starts up
  You can change it from Home screen > Menu (three-line icon in the upper right corner) > [Graphics].
• If you are using an Android device with a microSD inserted, check the location where the application is installed.

*If you confirm that the application is installed in the microSD, move it to your device's main storage.

For bugs and other issues, please feel free to check out List of Known Issues