The app may have become unstable.
Gameplay may become unstable when your device is overburdened, such as through extended play over long periods of time, which may cause your device to overheat. Make sure to take breaks while playing.
If the issue is still not resolved, try the following:
・Play in a location where the network connection is stable.
・If you're able to, turn on Power Saving from Menu, Settings, then System.
・Close apps other than the Mario Kart Tour game, including Internet browser apps.
・Restart your device.
In cases where the app is not working properly, check "Which devices are supported?" and confirm that your device meets the system requirements.