All mandatory fields of your ESTA application must be completed. Even though CBP have not explicitly stated that leaving a mandatory field blank will result in denial, it is a highly probable consequence, since omitting any of the mandatory fields will render your ESTA application invalid.
Furthermore, such an action may require manual adjudication, which will result in slowing down your application response.
Following is a list of the most common ESTA denial reasons:
Such eligibility questions help CBP determine whether or not you require a visa to enter the United States.
Note: Should your ESTA application be denied, you may still be able to travel to the U.S. In this case, apply for a nonimmigrant travel visa, class B1 or B2.