This error is displayed when there is an incomplete purchase.
Complete the purchase by following these steps:
1. Start the Mario Kart Tour game.
2. Tap Shop at the bottom of the screen.
3. Scroll to the bottom of the page and tap Complete Purchase.
If tapping Complete Purchase does not solve the issue, try reinstalling the app.
You will need to link your Nintendo Account when reinstalling, so check your Nintendo Account credentials beforehand.