Did you get a 1099Misc for something? Did you fill out Schedule C for it? Or did you report it as other income? Maybe the IRS got another 1099Misc you didn't get or report.
Do you have a Net Profit on Schedule C? That should give you self employment tax,
If you had a net profit from your self-employment then there would have been a Schedule SE for self-employment taxes. The amount of SE taxes from the Schedule SE would have been entered on Schedule 2 Line 4. The amount from Schedule 2 would flow to your personal tax return Form 1040 Line 15.
The IRS notice would have given you an explanation why the IRS changed your filed tax return to arrive at the Self-Employment taxes owed.
Look at your tax return.
To access your current or prior year online tax returns sign onto the TurboTax website with the User ID you used to create the account - https://myturbotax.intuit.com/
When you sign onto your online account and land on the Tax Home web page, scroll down and click on Add a state.
This will take you back to the 2019 online tax return.
Click on Tax Tools on the left side of the online program screen. Then click on Print Center. Then click on Print, save or preview this year's return. Choose the option Include government and TurboTax worksheets
Turbo Tax is a self prepared return. It has no way of knowing or checking that you entered something wrong or in the wrong place or entered something twice Or left something out. It doesn't check against any outside sources. The IRS has copies of all the W2s and 1099s your employer and banks/brokers, etc. sent in to match against. Turbo Tax only checks the calculations of what you entered. It is your responsibility to check it over before you file. They only Guarantee that the calculations are right based on the entries you made.