You need to file the missing 2017 Form 8606 to report the nondeductible contribution you made for 2017. The amount on line 14 of that form will transfer to line 2 of your 2018 Form 8606. In 2018 TurboTax, be sure to click the Continue button on the Your 1099-R Entries page and enter this basis when TurboTax asks if you made nondeductible contributions for years prior to 2018.
