Yes, you'll need to file a 2020 Form 8606 if the traditional IRA contribution for 2020 was not deducted on Schedule 1 of your 2020 tax return. This needs to be done separately from filing your 2021 tax return, so you should do it now. Although Form 8606 can be filed stand-alone, I would file it along with Form 1040-X to show that it has no effect on the rest of your 2020 tax return.
In 2021 TurboTax, indicate that you made nondeductible contributions enter $6,000 as the amount of your nondeductible traditional IRA contributions made for prior years, shown on your newly filed 2020 Form 8606 line 14. The result on the 2021 Form 8606 will be that your basis from 2020 will reduce the taxable amount of your Roth conversion.
Without the 2020 Form 8606 being filed, the IRS will not recognize the basis from your contribution for 2020.
