All mandatory fields of the ESTA application must be completed.
Although U.S. Customs and Border Protection (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 the application process.
The most common ESTA denial reasons are as follows:
Note: Should an ESTA application be denied, the traveler may still be able to travel to the US by applying for a nonimmigrant travel visa, class B1 or B2. However, this is a longer process, which involves going to a US embassy and may take weeks or even months to be approved.