I've just done it. Tried on the App but it rejected my credit card, same thing happened last September. My theory is that, when you register your card, you don't have to put the 3 digit security code. Normally, when buying online, you do have to input this. That's my theory anyway.
Back to the subject, I originally tried live chat but it referred me to having to telephone Princess to get it added.
After much holding on whilst I was transferred from one agent to another, I did manage to purchase the whole cruise, one device plan for the 50% discount. My advice, therefore, is to ring the number on the webpage.