RRB1099R: There is no box 2a nor 2b. The software asks if that's what the 1099R form states. But RRB1099R doesn't have Box 2a and 2b. What should I put in?

You are entering the form in the wrong place or checked the wrong type of form.  An RRB-1099 is entered under social security.  An RRB-1099-R is entered under the Pensions, IRAs topic.

If you have an RRB-1099-R and are correctly trying to enter it under Pensions, then you must have chosen the wrong type of 1099-R on the very first screen when you start that interview.  Use the Back button to select the correct type of 1099-R.  

If you are trying to enter an RRB-1099 under Pensions instead of Social Security, you'll have to delete what you have entered there and go to the Social Security topic instead.