I had to pay 2020 back to 2018 state(MT) taxes last year-202. Where do I report this?

In the Deductions & Credits section in your Federal return, scroll down to 'Estimates and Other Taxes Paid'.

You may have to click on 'Show More Tax Breaks' if you don't see this in the topic list.

Click on 'Income Taxes Paid'.

Scroll down to 'Other Income Taxes Paid in 2021'.