It is actually very common for a broker/administrator to check the box
"taxable amount not determined" and not have taxable amount filled out.
You don't have to determine on your own the taxable amount, TurboTax will help you with this. However, you do have to enter each form 1099-R into the program exactly as it appears -
do not make any changes to any of the boxes. Based on your 1099-R data entry the follow up answers
that the program asked you - taxable amount will be determined.