When you pay, the card is first checked with a £0 authorisation. Your form is then validated, and only then is the actual amount charged to the authorised card. That's why you may briefly see £0 or a £0 verification request from your bank.
If your bank declines the £0 check
Some banks block zero-amount checks. When that happens the payment stops before the real amount is attempted. Nothing is wrong with your entry or the price.
Try a different card.
Use Apple Pay or Google Pay.
Approve the verification in your banking app if prompted.
Note: For organisers: if a participant reports this, it is not a setup or pricing issue with your event. You can share this article with them.