Presumably you made your first Roth IRA contribution for some year prior to 2016. Because you were age 59½ or over in 2020 when you received the distribution (code T or Q in box 7 of the Form 1099-R), the distribution is a qualified distribution. Since it is a qualified distribution, you do not need to enter your basis in Roth IRA contributions or conversions. No reporting on Form 8606 is required.
