I've been getting this message when trying to apply a £5 for 2 months Cinema pass offer through my Chromecast. Been trying this for 2 weeks now and get the same answer all the time.
How can I redeem this offer??
Have you tried to apply the voucher promo code manualy when using the Chrome internet browser on your device ?
More info here on how Chromecast owners applied the cinema voucher code manualy to their NowTV account.