My 1099R does not have boxes 7a or 7b but the software won't let me continue without choosing a code. How do I move ahead?

There has to be a box 7.  It won't say the a or b, just 7.  And it probably has a 7 for the Code.  7 is a common code.  If you don't have a regular looking 1099R you need to ask the plan.  

Does it look like this one?  https://www.irs.gov/pub/irs-pdf/f1099r.pdf

The input screen has a box 7a and 7b in case there are 2 codes in box 7.  If you only have 1 code just use the first box 7a.