How do I enter a repayment to the US Treasury

repayment for what? generally, they're not reported as income on tax returns, and any interest paid that was included is generally not deductible. 

What was the repayment?  Did you pay taxes due from an audit or you just owed the IRS but received a form 1099 - Int?  If you mean you owed additional taxes and you paid them it is not deductible.  

The form 1099INT is includable in income.  Here is how to enter the form 1099INT.

Form 1099-INT reports interest from banks, brokerages, and other financial institutions. Here's how to enter a 1099-INT:

1. Sign in to your TurboTax account and open or continue your return
2. Select Federal from the menu, then Wages & Income
3. Locate the section titled Investments & Savings and select Show More, then select Start or Revisit next to Interest on 1099-INT
4. Select Yes on the screen Did you receive any investment income? then Continue
   • If you see the screen Your investments & savings or Here's your 1099-INT info, select Add investments or Add another 1099-INT, then Continue
5. On the next screen, select how you want to enter your Form 1099-INT:
   • If you want to import your form, search for your brokerage or financial institution. Sign in to your online account and select Get my form
   • If you want to upload a copy of your form or manually enter the info, select Enter a different way, and follow the instructions to enter your 1099-INT
6. On the screen Do any of these uncommon situations apply? check the boxes according to your situation and Continue