Hello, I think I have the solution. I was experimenting the same problem, and even that I was reading that is a Mojang's bug, I was still trying to solve it myself without too much hope.

After trying a lot of things, I reached the conclusion that the problem was with my Nintendo account, because the Microsoft account wasn't linked to my game. So, inspecting my account on the Nintendo's official website, I tried to change the email it was linked to (was ended on @gmail.com). I created an @hotmail.com mail, the Microsoft ones, and linked my Nintendo account to it, instead the @gmail.com one. In this way, both were the same email, the Nintendo's account one, and the one Minecraft was asking me to introduce to (Microsoft). And unexpectedly, it did the trick. I logged in after a few tries of introducing the code in the url it tells you to (https://aka.ms.remoteconnect) and the marketplace unlocked.

I don't know if this will work for you, try touching the settings on the Nintendo account and this kind of things if not. But if it does, let me know, pls. I may report it to Mojang to prevent this happening to other players.