If the letter has a couple of letters followed by numbers, you can refer to the following TurboTax Help article.
https://ttlc.intuit.com/community/irs/help/i-got-a-letter-or-notice-from-the-irs/01/27100
You can also visit the TurboTax Audit Support Center (link below).
https://support.turbotax.intuit.com/irs-notice/audit-support/
