I paid an additional $6,000 in back taxes in 2021. Where do I claim the additional taxes paid?

I HAD TO PAY BACK TAX OWED 

The money you paid to the IRS for a previous year’s federal tax return is not deductible, so you do not enter it anywhere on your 2021 return.  If you paid taxes due in 2020 to your state or local government, you can enter that by going to Federal> Deductions and Credits> Estimates and Other Taxes Paid> Other Income Taxes.  You can enter the taxes you paid, but do not include any penalties or interest.

There are six states that allow you to enter the federal tax you paid on your state return--Alabama, Iowa. Louisiana, Missouri, Montana, and Oregon.  If you are filing in one of these states watch for the deduction when you prepare your state return.