When the system sends a payment link to a customer and they click on the link before we allow the customer to enter their payment details we make a few checks first to make sure the payment request is still valid.
If your customer receives the following message; one of a few things may have occurred.
The booking had been cancelled previously and then the payment was marked as cancelled - the booking may have been then un-cancelled.
You may have made a modification to the booking which caused the initial payment request to be cancelled - a new payment request may have been sent to the customer. They will need to locate this new payment link or you can resend them the confirmation or receipt/invoice email, as this will contain the most up to date payment link.
Someone on your team may have cancelled the payment request on purpose or accidentally by clicking on the Delete button next to the payment request in the Payments Panel.