A required field is empty. Look for a red warning box and complete it. The charity selection is a common one.
A required checkbox (waiver, terms or declaration) isn't ticked.
The page is showing an old version. Refresh the page or clear your browser cache and cookies.
Your bank declined the £0 security check. See "Why does my payment show £0?".
Still stuck? Send a screenshot of the whole form to the organiser or to Race Space support.