Wont allow me to login

Using the App Store on Apple TV (4th generation or later), requires your Apple ID credentials for each app. It will default to the account information as set up in Settings﹥Accounts﹥iTunes Store and App Store﹥. It may also require a full details for the account, such as billing address and payment method. Setting payment method to ‘None’ is valid as long as you use free apps or have Apple ID account balance (‘store credit’). If the financial institution for a card requests the CVC from Apple, then that part may also be needed to be filled in. Change or remove your Apple ID payment information - Apple Support

What is your model Apple TV?

Identify your Apple TV model - Apple Support

Is this for a specific content provider?

Just setup my AppleTV and having the same issue. Whenever I attempt to 'Get' app (e.g. Netflix), I have to enter my password for my iTunes/Apple account (because it recognises my username) but after entering it, a dialogue screen appears with 'No iTunes Store Account' and asks to create one first. I definitely have one because I can log into iTunes on my phone and PC, and also to allow iCloud sharing on my new AppleTV I have to enter my account details and at this point it recognises my username and password.