When I try to sign in, I get a message saying "The email address or password is incorrect.", even after entering the correct email address and password.
It may be that you are entering the sign-in information in the wrong place.
If you have created a Nintendo Account linked to a Nintendo Network ID or a social media account, you will have to sign in by selecting the icon for the account your Nintendo Account is linked to, in the "Sign in with" section of the sign-in screen.
If you have forgotten your Nintendo Account password, please access the Nintendo Account sign-in page, click on "Forgot your password?" and then follow the instructions to reset your password.
If your account is linked to an NNID, you will need the password for the NNID. If you have forgotten the NNID password, you can check it here: https://id.nintendo.net/login.