Entry of the Form 1099-R itself has nothing to do with it. Code P simply means "taxable on the tax return for the year prior to the year of the Form 1099-R," but 2020 TurboTax's drop-down text just assumes that you are entering a 2020 Form 1099-R.
Proceed through the Traditional and Roth IRA Contributions section indicate that you have a Roth IRA, indicate that you opened a Roth IRA before 2020, then correct whatever is shown as the amount of your excess Roth IRA contributions for prior years.
