Nintendo Switch firmware update 10.0.4 is out now and fixes an issue which was preventing users in certain regions from registering credit card information on their accounts to purchase things on the eShop.
The full patch notes are below:
Ver. 10.0.4 (Released June 4, 2020)
Fixed the following issue:
We have fixed the issue where it was not possible in some countries or regions to add funds using a new credit card or to register credit card information in Nintendo eShop on Nintendo Switch.