1099-R’s come in two versions.
In the first version both box 1 and box 2a contain #s. This means that the plan custodian had the information to determine the taxable amount and did so. (Box 2b should NOT be checked in this version on your 1099.) Enter both #’s, (even if they are the same), and TT will put the amounts on either Lines 12a & b of the 1040A or Lines 16a & b of the 1040.
In the second version, box 2a is blank and, IF there is a box 2b, it will be checked. This means that the plan custodian did not have the information needed to determine the taxable amount. Leave box 2a blank, do not enter "0". Regardless of whether or not your 1099-R has the checkbox, you should check it on the TT screen since that tells TT that you left box 2a blank on purpose.
Based on your answers to all of the program's questions TT will compute the "box 2a" taxable amount and transfer it to the 1040 or 1040A. You don't need to attempt to calculate the taxable amount yourself.