If you are using the TurboTax online editions, to get Audit Defense you would need to purchase the optional Max Benefits
See this TurboTax support FAQ for how to get Max Benefits - https://ttlc.intuit.com/turbotax-support/en-us/help-article/intuit-product-orders/get-max/L9ZW1BYc5_US_en_US?uid=m9frleju
If you are using the TurboTax desktop editions, the option to purchase Audit Defense will be in the File section of the program.
Audit Defense is a third-party add-on service provided, for an additional fee, by TaxResources, Inc., dba Tax Audit.
Audit Defense is included at no added cost with business returns filed with TurboTax Live Business (excluding Sole Proprietor).