i'm getting the same error (on a 3ds), it's probably a pretendo problem. To be honest, this game's online didn't even work when the 3DS servers were active and working for games like mario kart and pokemon sun.

if you are on 3DS, it lets you play 100 mario challenge, but its not suported on wii u yet. pretendo has a discord and you can check for updates there.

I see, thanks for letting me know. I thought somethingwas acting up with my internet.