It won't let me sign in my Microsoft account in Minecraft on my Nintendo Switch.

Thank you, MatiasEhuan. In this case, you may need to reach out to the game support which is Mojang so they can have it checked for you.

Here you go: https://help.minecraft.net/hc/en-us/requests/new

Thank you for that. Can you sign in to https://account.microsoft.com without any issues?

Every time I press the, "Sign in with a Microsoft Account" it shows,"Signing into Microsoft Account". But the problem is that it shows that for a long time & doesn't completely sign in my Microsoft Account.

Yes, I can sign in to https://account.microsoft.com without any issues.

Hi, I hope you're doing great out there despite the issue that you're experiencing. I am an Independent advisor and I am here to help you in regards to your concern.

If I may ask, what is the error that you're getting whenever you're trying to sign in to your Microsoft account on your Nintendo switch?

By the way, please check and sign in to https://account.microsoft.com to see if you will encounter any issue just to validate.