I had to pay taxes to the state for 2018 in 2021. I underpaid it then. How do I include that on my 2021 federal return?

[xmasbaby0]

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 back taxes due in 2021 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.

[DoninGA]

To enter state income taxes paid -

• Click on Federal Taxes (Personal using Home and Business)
• Click on Deductions and Credits
• Click on I'll choose what I work on (if shown)
• Scroll down to Estimates and Other Taxes Paid
• On Other Income Taxes, click on the start or update button

On the next screen scroll down to Other Income Taxes Paid in 2021

Or enter state taxes paid in the Search box located in the upper right of the program screen. Click on Jump to state taxes paid