How did you attempt to apply the voucher code to your account, was it going to the nowtv website with your internet browser and clicking on My Account > My Passes under entertainment pass ?
If so, maybe try a different internet browser when adding the pass to your account.
Alternatively have you try applying the pass directly on your nowtv box by going into the nowtv App under My Account.
The nowtv scratch card codes that I have used in the past that come bundled with the box normally start with NOW********* and are usually around 14 digits.
Should have added, that if you are still getting no joy then jump over to live chat who will confirm if the code is valid or not.
If the code is not valid, then live chat may refer you back to Maplins for a refund or replacement.
Here's a link to live chat, where if you click on the drop down arrow below Anything else will reveal the live chat button.
I just want to add a couple of things to Schnapps advice, you said, "..... when i tried to add the pass it was 12 xx s and was told it was invalid....".
Well, 12 x's sounds very strange and at this point I would say that the retailer, i.e. Maplins, is responsible to supply you with a valid pass. So after takings Schnapps advice, and if it doesn't work then bear in mind you might have to take your Now TV kit back to Maplins. And don't let them fob you off.
However, as Schnapps said, the pass should be in the form of a scratch card which reveals the pass code when the silver covering is scratched off. At this point, as a personal policy, I always get in touch with Now TV's help chat to confirm that the code is indeed valid.
Then click on the "Anything else?" down arrow to get to the chat screen.
Hope this helps.
Another thought did the nowtv box also come supplied with a Sky Store voucher which you have mistakenly trying to add to your nowtv account ?
I have seen some of the newer supplied boxes where the nowtv voucher code is located on a scratch panel inside a nowtv booklet rather than on a single card.
Also, make sure you are entering the nowtv voucher details correctly where things like O & 0 etc can cause an invalid error.