After configuring Service Catalog to allow Users to Cancel Requests, the button is missing in the Request Detail form.
The configuration is done in: Catalog Tab > Configuration > Request Management Configuration > Access Control: Cancel Request > Select <End User>
The option Catalog Tab > Configuration > Request Management Configuration > Allow Cancellation Through is set to Submitted.
Set Catalog Tab > Configuration > Request Management Configuration > Allow Cancellation Through to Fulfilled or Approval Done.