I am also having this issue. I contacted Nintendo’s live chat and they said they couldn't help me, and to ask Epic Games for some weird reason. I asked if they meant Microsoft and they said yeah, so I posted on the only Microsoft forum I could find that was somewhat related, and they told me to ask Nintendo, and just gave me the same links I already found that don't solve the issue.
I saw a comment somewhere that I think may be the issue- It's stuck in a loop. It thinks you need to be logged in to a Microsoft account to play any multiplayer even local, but when you change to Local network it logs you out of Microsoft, but if you're logged in then it thinks thinks you need Nintendo Online. So even though neither of these have been a requirement before it seems to think they are now for some reason.
Elsiegurl
I am also having this issue. I contacted Nintendo’s live chat and they said they couldn't help me, and to ask Epic Games for some weird reason. I asked if they meant Microsoft and they said yeah, so I posted on the only Microsoft forum I could find that was somewhat related, and they told me to ask Nintendo, and just gave me the same links I already found that don't solve the issue.
I saw a comment somewhere that I think may be the issue- It's stuck in a loop. It thinks you need to be logged in to a Microsoft account to play any multiplayer even local, but when you change to Local network it logs you out of Microsoft, but if you're logged in then it thinks thinks you need Nintendo Online. So even though neither of these have been a requirement before it seems to think they are now for some reason.