I owed taxes for 2018 and paid them in 2021. Do I enter this somewhere in my 2021 taxes?

[Kat]

No.  There is no entry or credit for past-due taxes on your 2021 federal return.

[ColeenD3]

Federal income taxes are never deductible. If you paid prior year property taxes, include them on Schedule A, with your normal property taxes. If you paid prior year state income taxes, again include them on Schedule A. 

State tax deductions are limited. Starting with the 2018 tax year, the maximum SALT deduction became $10,000. There was previously no limit.