Step 1 : Create a report.

In the reports tab -> press the "+" button.

Step 2 : Generate the report.

Select "Unpaid bookings (appointments)" and the fields you want to make appear in the report.

Choose the dates and click on generate.

This way you will have a report on all the people who did not pay their appointment. As soon as an unpaid appointment is paid it won't appear anymore is this report.