Is for the full amount you paid? When did you pay it? If it was after July 15 it might be for interest. Or if you owed too much it might be a penalty for not paying in enough during the year.
How did you pay it? Did you send in a check? Did you use the 1040V voucher? If you used a 1040ES slip that was to make an estimated prepayment for NEXT year 2020 return.
Your only proof of payment is your bank or credit card statement showing it coming out. Turbo Tax does not pay it. They only send your bank info to the IRS or state and then the state pulls it out of your account.
Or the IRS changed something on your return or added some income. The letter should say what they changed.
