Assuming that you are referring to a 2019 tax return that has already been filed. You had to purchase the Max Benefits package, which includes Audit Defense, before you filed the 2019 tax return in the File section of the program on Step 1.
If you purchased Max Benefits you would have received an email for how to access Audit Defense. See this TurboTax support FAQ - https://ttlc.intuit.com/community/using-turbotax/help/when-will-i-get-my-audit-defense-membership-in...
If you did not purchase the Max Benefits package you can use the TurboTax Audit Support Center for assistance with an IRS letter or notice you received. The Audit Support Center can provide assistance in understanding the IRS notice or letter and provide you with steps on how to proceed.
Support is open from 5am to 5pm Pacific time, Monday thru Friday
Go to the Audit Support Center website for assistance during business hours - https://support.turbotax.intuit.com/irs-notice/audit-support/