If you already filed your return it is too late to purchase MAX
https://ttlc.intuit.com/community/choosing-a-product/help/what-is-max/00/26700
TurboTax does not receive any information from the IRS or your state after your return was filed, so no one at TurboTax knows about a letter you received from the IRS or the state.
(what the letters from the IRS mean)
I GOT A LETTER FROM THE IRS
https://ttlc.intuit.com/questions/3605673-i-got-a-letter-or-notice-from-the-irs
I AM BEING AUDITED
If you purchased Audit Defense:
https://ttlc.intuit.com/questions/2924451-i-bought-max-or-audit-defense-and-now-i-m-being-audited
AUDIT SUPPORT
TurboTax has a special phone number for help with IRS/state tax letters, which will be easier than going through regular Customer Support. At the page below, click on the blue button that says "Get Help from TurboTax Support". Then it will ask you what tax year is your letter. Then it will ask you what the letter concerns. Then it will show you the phone number during posted business hours.
https://support.turbotax.intuit.com/irs-notice/audit-support/
As xmasbaby mentioned, you can get free help with an IRS letter at the TurboTax free Audit Support Center at the link provided above. When you go through the "Get Help" process there, it will provide you a phone number.
In addition, you appear to be using Live Premier. You paid extra for that Live feature. That product has live help from a CPA/EA tax expert that you can ask for by phoning TurboTax Customer Support.
Although you can't purchase MAX (a suite of several services) after filing, it is possible to purchase standalone Audit Defense after filing directly from the company that provides it or to pay them to help with an existing IRS problem.
But I would try to exhaust all the free options you have first with the TurboTax free Audit Support Center or your Live help from a TurboTax CPA/EA expert.
If you mention what the IRS is claiming in the letter, you might get help/suggestions from other users in this forum as well. But don't post any personal info. This is a public Internet forum.