I signed up this website specifically to reply to this question. Cause I myself experience this frustration for days.
I knew it's possible to buy tickets from Ryanair because I did it 1 month ago without any trouble.
Then I tried to book a long distant flight at home and it doesn't work. The website kept showing "Contact Info missing" or "Payment Error"
I tried different credit cards, different computers. Nothing works.
Then I went back to use the computer I used 1 month ago and with Mozilla Firefox, still fail. Until I tried Google Chrome, I finally got my tickets. 10 minutes ago!!!Yeah!!!
So the trick is you need to stick to Google Chrome, try on different computers, finally you will get one works. Good luck guys!