ezyVet shows an already approved on billed invoice error when you cancel an appointment
Do this troubleshooting if ezyVet shows You cannot disable this, it is aiready billed on approved invoice when you try to cancel an appointment:

The appointment has a related approved invoice
Unapprove the related invoice
- Find and open the invoice record that is related to the appointment.
- Unapprove the invoice.
- Try to cancel the appointment again.