Under certain circumstances your game may lag, crash, or doesn’t launch.
The lack of storage space might be the reason of this. To check available storage space, go to Settings. You can free some space by removing unused applications.
For optimal performance, it's important that you clear the memory of your device from time to time. You can do this by shutting down apps you're not using at the moment. When all applications have been closed, launch the game again.
Defenders 2 might not be supported by your device.
Try rebooting your device and/or reinstall the game.