Have you applied a previous Merlin voucher promo offer to your cinema pass account before ?
More information here about the Merlin Entertainment NowTV Pass Offer.
If you haven't applied a Merlin voucher before to your cinema pass account, perhaps try using a different internet browser on your device when applying the voucher.
Try a different internet browser or device when applying the voucher code, if that fails contact live chat and ask them if they can check the code is valid and if they can offer you further assistance.
To contact live chat, open the link below and click on the drop down arrow below Anything else, where you should see an option to contact live chat.