If you are having trouble accessing any of the eShops, you are not alone. All of the digital stores are down at the moment.
The issues naturally started shortly after Super Smash Bros. Ultimate version 3.0.0 and the Joker DLC went live, so you may also be unable to download the latest patch. Fortunately, Nintendo is aware of the situation and is working to get things back up and running. The company says it will provide an update as soon as it has more information.