Code J indicates that you received a distribution from a Roth IRA, not from a Roth 401(k). Box 5 of the Form 1099-R has nothing to do with a distribution from a Roth IRA.
You must enter the Form 1099-R as received. Be sure to click the Continue button on the page that lists the Forms 1099-R that you have entered, then, when asked, enter your basis in Roth IRA contributions and Roth conversions.
If the money was originally in a Roth 401(k) and was subsequently rolled over to a Roth IRA, you should have received and reported a code-H Form 1099-R for the year in which the rollover occurred. The amount in box 5 of that Form 1099-R would have become contribution basis in your Roth IRA.
