Welcome to the forum.
It may be because you didn't enter your payment details when you signed up, even though you have a voucher you need to enter a valid payment card on your account to verify your identity and act as a payment source for any future purchases.
Try going to http://www.nowtv.com/activate and clicking on the Complete Account Setup button, using the same username and password you've already set up.
Just to check, when you select the Complete Account Setup button it should take you to a page where you can either sign in with username and password, or click a Join Now button to sign up from scratch.
Make sure instead of clicking Join Now, you enter the username and password you've already chosen earlier into the top section and click Sign In. Then it should check your account, and prompt you to enter any details that are missing such as payment card.
It might be easier for you to have a little word with Live Chat, they can try and see what is stopping your account from being fully setup, and it will save you having to keep posting responses here.
To get there go to the following help page
and click the green Contact Us button near the bottom left of it (not the How to Contact Us link right at the bottom as that takes you somewhere else) then choose the Live Chat option on the following page.
Hope you get sorted 🙂