Click the Continue button on the Your 1099-R Entries page after entering your Forms 1099-R. When TurboTax asks if you made nondeductible contributions to your traditional IRAs, answer Yes, then answer the additional questions regarding the amount of your basis in nondeductible traditional IRA contributions and the 2018 year-end balance in your traditional IRAs. TurboTax will prepare Form 8606 to calculate the taxable amount of the Roth conversion.
Also make sure that you've correctly marked the IRA/SEP/SIMPLE box in TurboTax as marked on the Forms 1099-R provided by the payer(s). Only distributions from traditional IRAs have this box marked and only conversions from traditional IRAs are reported on Parts I and II of Form 8606.
