On my tablet, I filled in all the details - name, full address, email (twice), phone no, then the names (first and Sur) for both tickets, then clicked continue.
It threw up an error saying "please fill in all fields", or words to that effect and it highlighted my title, my email and my country (UK) in red, which made me think there was something wrong with those.
I re-entered these things a number of times, to my great frustration, but eventually accidentally scrolled down a wee bit and saw that the card payment boxes were on the same page, and it was those that it was, quite reasonably, referring to as incomplete!
Perhaps on another device I would have seen these were part of the same screen, but the continue button that I could see led me to believe that payment details would be on a subsequent screen.
I am quite old, and I only worked in IT for 21 years, so what do I know?
Frustrating.....