As Mike says, Part II of the 8889 deals with distributions from your HSA.
My guess is that you spent money from your HSA in 2020, and so your HSA custodian sent a copy of the 1099-SA reporting this distribution to you and the IRS...however, you didn't get your copy, and so did not enter it into TurboTax.
If you did not enter your 1099-SA into TurboTax, then you would not have answered the question, "were these distributions all for medical expenses?". If not, then all the distributions that the IRS saw would be added to your income, causing you to owe tax, penalties, and interest.
Like Mike, I suggest that you contact a local Enrolled Agent or CPA to go over this problem with you and represent you (this likely means that they would respond to that letter on your behalf). You will not have to go see the IRS, this is something can most likely be handled with mail. Your EA or CPA will have you contact your HSA custodian to ask for a copy of the 2020 1099-SA, and if you can't get it from them, the EA/CPA will know another way to get it.
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