Adding to this thread as it appears nearest to my problem. Code BB in Box 12 (403(b) Roth elective deferral).

Had to go all the way down to Part X of Federal Information Worksheet to section entitled "Credit for Qualified Retirement Savings Contributions (Form 8880)" to check No on whether taxpayer was a full-time student (even though it was showing NO on 8880 already).

Now 8880 populating.