Ummm maybe because you havnt used that Apple ID with the iTunes Store yet ??
You need to give us more information, what Mac, what OS on the Mac, what .v iTunes on the Mac, what have you done i.e. : signed out of iTunes ?
iTunes is heading towards a 2 way verification and if you have recently signed out, it might be saying this so you go through the process of telling Apple you are who you say, and one way to do this is to sign in update your account ufo, OR verify your payment details, or the secret questions.
Apple lately is taking the security of your Apple ID very seriously, so if there are prompts, follow through them and you should be back on track.