My test step is continually running without any results. Or, you're getting a "Problem creating Webhook Subscription" error.
In order to use Calendly with Zapier you must have a premium/paid Calendly account, as noted here: https://help.calendly.com/hc/en-us/articles/235289507-Zapier
I need to trigger on rescheduled appointments
When an appointment is rescheduled on Calendly, the mechanics of that action are such that the original appointment is canceled and a new appointment is created. Depending on what you're looking to accomplish, you can use multiple Zaps with both "Invitee Created" and "Invitee Canceled" triggers to achieve your desired workflow.