Jump onto live chat and speak to staff. Make sure you are signed in and use the 'How to get in touch' drop down.
Do you have an active Boost membership?
The £1 for 3 months cinema offer usually includes 1 month free Boost, which if you already have an active Boost offer that is not in the final 30 days causes an error. If that is the case this is the work around I have used before:
1. Log out of nowtv.com
2. Click the 'Claim Offer' button on the offer email
3. The 'basket' that loads will show 2 items; 'Cinema Membership' £1.00 and 'Boost' £0.00
4. Under 'Boost' click 'Remove Voucher'
5. 'Boost' will now show at full price of £6.00, but worry not, simply click 'Remove' to delete it entirely
6. Finally over to the right of the page click 'Already a member?' and sign in and confirm payment.
Sorry I can't have explained properly, when it gets to already a member, I can;t sign it and it says technical difficulties, I went to the chat today and it told me to update my bank card - but I pay by dd.
What happens if you try a different internet browser ?
When you click on the email link offer is there any NOW voucher code embedded at the end of the url address bar in your browser?
Usually it will start with NOWxxxxxxxxxx and be a combination of letters and numbers around 12 to 14 digits long.
If there is a NOW voucher code, then copy it down and try applying it directly to your NOW online account under My Membership where you see the main apply voucher box.
I was after this particular offer myself, but was unlucky not to get it (even though I have no active Cinema Membership running on my account).
Anyway hope you get it sorted (assuming the expiry date to claim this offer is still valid on the email T&Cs).