You must click the Continue button on the Your 1099-R Entries page, then, when asked, indicate in 2018 TurboTax that your first Roth IRA contribution was for some year prior to 2014. TurboTax will then show the distribution as nontaxable income.
You can also tell TurboTax your contribution and conversion basis, but this is unnecessary to determine that the code T distribution is nontaxable as long as you've indicated that your first Roth IRA contribution was for some year prior to 2014.
