Some letters are posted in your IRS account. If you have not created an account yet, you can do so by going on the link below. If it is not available in your account you will have to wait 2–3 weeks to receive the letter or call the IRS directly.
Link to create an account with the IRS
https://www.irs.gov/payments/online-account-for-individuals
Link to contact the IRS
https://www.irs.gov/help/let-us-help-you