An order line with its delivery method set to Delivery cannot be scheduled unless a service work order has been created for that line.
By default, if a service work order has been created, the line can be scheduled after an appointment has been taken. If the appointment has not been taken and a scheduling attempt is made, the line is dropped in reserved status.
However, Sterling Selling and Fulfillment Foundation can be configured to allow orders to be scheduled before service work order appointments are taken.