Enter the Form 1099-R exactly as received. Box 2a should be the same as box 1 and box 2b Taxable amount not determined will be marked.
After entering your Forms 1099-R, click the Continue button on the Your 1099-R Entries page. TurboTax will prompt you to enter or confirm the amount of nondeductible traditional IRA contributions that you made for years prior to 2016. After you enter this, TurboTax will prepare Form 8606 Part I to calculate the taxable amount of your distribution.