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 in which the app is not working properly, check the section "Which devices are supported?" under System Questions.