Is the TicketResultError field only for the cancelled/voided tickets?

It is for any error that may happen based on your request. One of the following reasons can be thrown if the ticket has any error:

enum ReasonCode {
  // Not a valid value. Used for blocking 0 as invalid state.
  REASON_CODE_UNSPECIFIED = 0;
  REASON_CODE_INVALID_ARGUMENT = 1;
  REASON_CODE_ALREADY_EXISTS = 2;
  REASON_CODE_NOT_FOUND = 3;
  REASON_CODE_UNAVAILABLE = 4;
  REASON_CODE_INTERNAL = 5;