I tried to create a Nintendo Account, but I got a message saying ''The email address you have entered is already registered. Registration cannot be completed.''
Only a single email address can be linked to one Nintendo Account. If the email address you have entered is already registered, it cannot be used again.
Alternatively, this can occur if you are trying to sign in with a Nintendo Network ID. If you have multiple NNIDs registered to one email address, you will only be able to create a Nintendo Account with one of those NNIDs.