If you got a Notice of Deficiency, you have already failed to respond to several earlier notices. You should probably find a U.S. tax professional in whatever country you are living in, and go to the tax professional for help. The tax professional will want to see a copy of your tax return and all IRS notices that you have received. You can search for a U.S. tax professional in a specific country using the Directory of Federal Tax Return Preparers on the IRS web site. I suggest that you search for an Enrolled Agent or a Certified Public Accountant.

If you purchased Audit Defense for your 2020 tax return, call TaxAudit at +1-877-829-9695, or report your IRS notice on the TaxAudit.com web site. Do not contact the IRS until you have spoken to TaxAudit. They may contact the IRS on your behalf.

If you did not purchase Audit Defense, you can get help with your IRS notice from TurboTax at the following link.

We can help you with your tax letter

READ the notice carefully so you know how/if you need to respond ...  https://www.irs.gov/individuals/understanding-your-cp3219a-notice

Can I trust taxaudit.com? If so I will try to contact them. Unfortunately, Turbotax call center is not helpful for international calls. Btw, I moved out of US in 2019 to Türkiye. Most likely that is the reason why I didn't receive any other notice. I would appreciate any other potential source of help.

A little bit more detail: The brokerage firm sent a 1099B to IRS of my exercised, company issued stock options as if I haven't reported it. Eventhough it was part of my W2 and the taxes were paid, now IRS is asking for full taxes on the exercised stock options based on that 1099B. I will appeal and need a professional to take care of it as I am outside the US.

Thanks for the help