There is a bug in Pokemon Duel that causes players to be stuck on the home screen. The buttons cannot be pressed and the bottom option bar is grayed out.
How-to Fix Non-Responsive Home Screen Issue in Pokemon Duel
You will need to force close the app and reopen. You may also need to turn off and on your wifi and restart your device. If the issue still occurs, hit the Clear Cache button on the title screen and try again.
Unfortunately, if none of these steps work you will need to wait it out. Do not uninstall the game without creating a Transfer ID, otherwise you will lose your account.