Ensure you are not using the test workflow button when testing the Facebook Conversion API in Workflow. Instead, save and publish your workflow and use one of the specified triggers to test your conversion.
The trigger should be an input trigger such as Form Submission, Survey Submission, Appointment Submission (from the booking widget), or a 2-step order form.
For appointments, the trigger that works is Customer Booked appointment and not just Appointment, as the latter is a general trigger. Make sure to use the specific trigger for the widget.
If you are using the Purchase event, the lead value is required. Otherwise, you can leave it blank.