The $20k taxable amount in box 2a is correct on this code G Form 1099-R reporting a direct rollover from a traditional account in a qualified plan to a Roth IRA.
Form 1040 line 16 is the correct line for reporting this rollover from the 403(b) to the Roth IRA. Since the entire amount being reported on line 16 is taxable, the correct indication on line 16 is a blank line 16a, $20k on line 16b and the word ROLLOVER next to the line.
If line 16b is not showing the $20k as taxable, edit this 1099-R form in TurboTax and make sure in the follow-up to indicate that you rolled the entire amount to a Roth IRA (not to a Roth 401(k)). If this doesn't cause the $20k to appear on line 16b with the ROLLOVER notation, delete and reenter the Form 1099-R.
