You can add a new voucher in the last month of your current voucher, so you should be able to add it now. Are you sure you are clicking in the right section (movies/ents)? What is it saying when you try to add it?
For info on adding vouchers see here
If you are getting a voucher invalid message, see here for some tips
@Anonymous User do you have a Cancel option. If you hit that it might make the voucher option appear (fortunately it won't end your pass early and will ensure you are definitely not charged in 5 days)
The cancellation page loads fine for me, have you tried again yet? Is there another browser you can try? The page should load fine on tablets and phones but might be best to try on laptop/desktop if you haven't already.
If you still have problems after that, might be worth having a word with live chat to see if they can get to the bottom of it. To get there go to the following help page
and click the green Contact Us button near the bottom left of it (not the How to Contact Us link right at the bottom as that takes you somewhere else) then choose the Live Chat option on the following page.
@Anonymous User To add on to what @Andy says, try deleting your cookies beforehand. It helped me.