This may occur in the following scenarios:
The product you are trying to add to your cart is a product that has restrictions on how many can be purchased by a single Nintendo Account, and you have already purchased the maximum quantity.
If, due to cancellations or other circumstances, the quantity of a limited item that you have purchased falls below the maximum amount allowed, it will still display as you having reached the limit until the cancellation is reflected in the system. In this scenario, please wait a while and then try again.
The product you are viewing is a Nintendo Switch Online Exclusive product and your Nintendo Account does not have an active Nintendo Switch Online membership. For any Nintendo Account that does not have an active Nintendo Switch Online membership, the Add to Cart button will not display. You can view Nintendo Switch Online membership options here.
When the product you are trying to add to the cart is out of stock.
When the product is a point exchange product, and you do not have enough points. You can check the points you've collected, the points you've redeemed and points which have expired by going to your My Nintendo Points History.
Also, if you get the error message "Invalid Form Key. Please refresh the page," please refresh your browser and try adding the product to your cart again.