Hello Me_873,
Welcome to the Microsoft Community!
Thank you for posting your question. Based on your question, I understand that when you connected your Nintendo device using your Microsoft account, you were notified that your account was already logged in on Minecraft.
I have determined that the reason for this notification is due to the fact that Microsoft accounts only allow the same account to be online on one device while playing. You are not allowed to use the same account to play on multiple devices at the same time.
Please recall if you have ever given your Microsoft account to someone else to borrow. If you want to continue to be associated with a Nintendo device, you will need to log out of your online game and try again. You can view and manage your online devices through the official website of your Microsoft account.
Hope this helps you. Looking forward to your reply we will also adjust and improve the solution for you based on your reply.
Best Regards,
Zhang Yang - MFST | Microsoft Community Support Specialist