Payment settings not accepting my new card details

30GB monthly data plan. Not accepting anything in assoicaited products

I am trying to update my card details so that I can pay my sunscription.
It is being rejected. The reason given is that the security card is invalid. I am entering the number as shown o the card. All other card details are correct.

I can't find a way to deltethe existing card (just in case that is causing the problem.

what do I need to do