You can't simply go to the My Nintendo Store and buy the Switch 2. To order the Switch 2, you need to log in with an active, paid Nintendo Switch Online account. Once you do that, you can choose either a Switch 2 standalone console or the Mario Kart World bundle and then hit the confirm button to "register your interest" in ordering a Switch 2.

The Nintendo Switch 2 is launching at a price of $449. There is a limited-time bundle that includes Mario Kart World that costs $499 (which, considering the game itself will retail for $79.99, is a good deal if you plan to buy Mario Kart anyway.)