I get this Error in red each time I click on Start offer, this is through the NowTV website from my PC, so not sure where I am supposed to enter a PIN?
What happens if you revert back to Chrome browser does the offer reappear?
Also have you created a Payment Pin on your NOW account, where it sounds like NOW are asking for your NOW payment pin (not Bank card pin).
Maybe go into the Payment Pin section and temporary disable it.
Could be a random issue with the website. It is worthwhile heading to live chat and explain the situation.
Just make sure to click “chat online”.