If you purchased Audit Defense or MAX last year, you should have gotten an email with your certificate and welcome kit. I suppose it's possible for an email not to show up or end up in a junk/spam folder.
If you used Online Turbotax, log in and in the section "Your Tax Returns & Documents", choose tax year 2017. Then choose a link to "View order details." Is there any evidence there that you purchased Audit Defense or MAX?
If you still have a question as to whether you purchased it or not, you can phone the Audit Defense folks at (intuit.taxaudit.com) as xmasbaby0 suggested above or TurboTax Audit Support Center.
If you did not purchase Audit Defense or MAX for 2017, you can still get free help from TurboTax free Audit Support Center as xmasbaby0 mentioned above. To get the phone number go to the page at the link below, click the blue "Get Help" button, and follow the prompts. That department may also be able to look up your account to see if you bought Audit Defense/MAX last year.
https://support.turbotax.intuit.com/irs-notice/audit-support/
Also, if you explain more about what the IRS is alleging in the letter, you may get additional comments in this user forum over the weekend. Arithmetic error for what?
