The IRS letter calls it Social Security tax, but it's referring to the Social Security portion of your self-employment tax. The CARES Act allows someone with self-employment income on their 2020 tax return to defer paying part of the Social Security tax that's included in the self-employment tax, if certain requirements are met. Whether it was intentional or not, you did defer paying it. The deferral appears as a refundable credit on Schedule 3 line 12e of your 2020 tax return. It was subtracted from the tax you had to pay with your tax return. The only way to "clear" it is to pay the tax that you deferred. Half the deferred tax is due December 31, 2021, and the remainder is due December 31, 2022.
TurboTax asked whether you wanted to defer the tax. If you clicked Yes it went on to calculate the maximum amount you could defer, and then asked you how much you wanted to defer.
