I got this too when trying to add my parent's email to the iPad mini. It is a Yahoo security feature.
What you can do is log in to Yahoo on a PC and it may say "you are trying to access your email via a device that has not previously been used. Please answer this security question." There should be a question such as What is your favourite pet's name? etc. Once you answer this, then I'm hoping I can add Yahoo Mail to the iPad mini. (I haven't yet as it's a present for my parents and they don't know until Christmas).