You may see this situation if you have both an Nokia Mail powered by Yahoo! account and a Yahoo! Mail account because you can only be logged into one of your accounts at a time since they are both running on the Yahoo! platform. 

Make sure that you have logged out of your Yahoo! Mail account, and then go to to log in to your Nokia Mail account. Alternatively, if you have previously associated your Nokia Mail address with a Yahoo! account, you will be taken to your Yahoo! Mail account.