Nintendo Switch error code 2618 0521. What should I do?

I've always had this mistake and finally want to get rid of it. What should I do every time I want to play a Nintendo server game online, it doesn't work, and this code comes with e.g. Mario kart 8 Deluxe online or super smash Bros or Animal crossing I really can't play anything except e.g. Brawlhalla or Paladins… Stop these games that are not from Nintendo.

Please help me…

https://en-americas-support.nintendo.com/...-2618-0521

Possible solutions are explained here.