As of 3rd September 2019, credit cards cannot be used to add funds on systems in the Nintendo 3DS family or Wii U consoles.
A new off-device top-up website will be launched for consumers who want to add funds using their credit card.
The off-device top-up website (https://ec.nintendo.com/my/balance_add) will be accessible by consumers from 3rd September, 2019.
How to add funds in Nintendo eShop Using Credit Cards:
Ensure that the Nintendo Network ID (NNID) on your system is linked to a Nintendo Account.
See here for more information on how to create a Nintendo Account and how to link a Nintendo Network ID to a Nintendo Account.
Visit https://ec.nintendo.com/my/balance_add select the payment method you would like to use and follow the on-screen prompts to complete the process.
Once you have added funds to your NNID this way, you can use them directly within Nintendo eShop on your system.