Debug Log Settings Available in BookingPress
From this section, you can enable/disable the payment logs. It will help to debug the actual issue while processing the payment. You can view, download and clear the logs from here.
When you enable the log for any particular payment gateway, the system will store all the logs in the database for all payment transactions. So, in the situation where you found out that the customer has already paid for the booking but the appointment is not booked for that customer.
In such cases, if you turn ON the logs, you can find out what is going wrong with the payment process etc.
So, when in doubt, always enable the logs to check what is actually causing the issue!