The instruction is correct. Where TurboTax is asking for your basis in nondeductible traditional IRA contributions made for 2019 and prior years, that's precisely what it wants you to enter, not any basis added by a traditional IRA contribution made for 2020. In essence, TurboTax is asking for the amount from line 14 of your most recent previously filed Form 8606. If the contribution you made in 2020 was for the 2019 tax year, that contribution would have been included on your 2019 Form 8606.
The basis for a traditional IRA contribution made for 2020 is added by entering your 2020 IRA contribution under Deductions & Credits -> Retirement and Investments -> Traditional and Roth IRA Contributions. If you included your contribution made for 2020 when TurboTax asked for your contribution basis from years prior to 2020, that has likely produced an erroneous Form 8606 with your contribution for 2020 being present on line 2 instead of on line 1.
