Open Settings, Invoices and find Cancellation charge. Choose None (the default), Flat amount (before tax) or Percentage of job price, and optionally Free cancellation until N hours before pickup (0 = never free). The pickup time is the job's Ready from time; jobs without one have no free window.
A customer can have its own rule: edit the customer and choose a Cancellation charge other than Company default. The charge is used only when the customer cancels; cancellations on your side are always free for the customer.