Have you tried a different internet browser when going to your online NowTV account, under My Account > Manage Account > My Passes and tried to apply the voucher code either by clicking the Apply voucher button underneath the Cinema Pass or clicking the green Redeem Voucher button towards the top of the screen page.
If that doesn't work, see if the voucher code can by entered directly on your Youview TalkTalk box by going into the NowTV App on the box, then go to the bottom of the home screen App page, highlight and click on My Account, where there is an Apply Voucher button to click on and add the code from there (assuming the TalkTalk voucher is similar to how the NowTV voucher codes are used on their service).
Based on the above i am assuming you are not on an existing Movie offer that has 30 days or more left remaining and you bank card details on the NowTV system is up to date.
This is a customer forum, so no-one from Now TV will contact you. They do occasionally monitor the forum however.
So I would recommend these steps.
Firstly check this link out to see how many accounts you have with the same payment details.
Secondly set up a payment PIN
Thirdly either pop onto Live chat to speak with Now TV staff about the situation.
Just make sure to click “chat online” within the green box.
Alternatively send an email to the address in the below picture.