If the app doesn't start and you're using a supported device, please try the following.
Note: Check Supported Devices to see which devices are supported.
・Play in a location where the network connection is stable.
・Close apps other than the Mario Kart Tour game, including Internet browser apps.
・Restart your device.
・Make sure you have enough available storage. If you don't have enough storage, consider deleting unused apps and data.
・Reinstall the Mario Kart Tour game. You will need to link your Nintendo Account when reinstalling, so check your Nintendo Account credentials beforehand. I want to check the e-mail address registered with my Nintendo Account.