To switch to Paypal payment you need to be an existing customer according to this linked help article below.
With regards to using your bank card to create a NowTV account, the card needs to have either the Visa or MasterCard logo on it, the card needs to be from a UK registered bank where you are a UK resident 18 years old or over and have sufficient funds in your account.
I have come across people who have attempted to use up front pre paid credit cards setting up a new account encountering difficulties with the nowtv online system.
See if NowTV live chat can assist you further.
Open the link page below and select the most appropriate help topic, then once you get to step 3, scroll down to the bottom of your screen page, where hopefully you will see Get in Touch (click on the drop down arrow below it to reveal the live chat button).
Soon as my contract has finished I’m going to another provider, every month I have to ask someone to pay my account,even though I have funds to pay it for what ever reason ,I have had many conversations with now tv about this problem today I have to transfer money to my friend and she pays it for me ,I have Never had this problem with any provider Ever ,I can’t wait to leave my contract.