I have the same issue. My voucher expires 19th Jan and there is no Apply a Voucher link. When I apply the voucher on the link above I get the same issue:
"It looks like you're already on an offer for the same Pass as the one that voucher's for. Come back in the last 30 days of your offer to use this voucher."
The strange thing is when I was cancelling the Entertainment Pass, they offered me 3 Months FREE Cinema pass and I of course accepted it. But when I checked in my account , they were going to charge me £4.80 per month for 3 months INSTEAD OF £0.00. So I cancelled it straightaway and now I can't add another voucher. Which makes me believe even though I cancelled I will be charged next month. And CS doesn't believe me that I have been offered FREE 3 Months Cinema Pass. And I can see I am not the only one. There is definitely something wrong with that offer, but no one seems to care.