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That message usually means the account you are signing into on your iPad isn't fully set up yet.
This be because you've accidentally entered the wrong username and password, in which case check you are entering the right details - you might want to log into the website first and go to your My Account > My Passes page to make sure your pass is there - if it is you know the login details are correct.
Or it could mean you've set up an account but didn't finish it properly - did you enter your payment card details as you need to do this to finish your account properly. If you don't think you have, go to nowtv.com/activate and click the complete account setup button, then sign in with your username and password to finish setting up your account.