If you made a nondeductible traditional IRA contribution for 2018, you must file 2018 Form 8606 to report that, otherwise the IRS does not have to recognize that as having added to your basis in nondeductible contributions. Line 14 of that form will have the amount that 2019 TurboTax is asking for as your basis from years prior to 2019 and will put that amount on line 2 of the 2019 Form 8606 to be used in calculating the taxable amount of your 2019 distribution. If you made a 2019 traditional IRA contribution, you'll need to enter than separately in 2019 TurboTax.
