Hey, same thing was happening to me, but I found a video on youtube which fixed it for me. Just search up How to fix Splatoon error code 104-2210. The video is by johnboshi. Even if your error code is different, it should still fix your problem. God Bless!

The main way that I've seen fix this error is to go into your Data Management settings and delete "Shared Save Data" (or something close to that name) and reboot the game. Hope that works for you!