Are you sure that this Form 1099-R is for a distribution from a Roth IRA. Th code B in box 7 indicates that this distribution was not from a Roth IRA but was instead a distribution from a designated Roth account in an employer-provided retirement plan (which is reportable on Form 1040 lines 5a and 5b). If it was truly from a Roth IRA, the payer used the incorrect code in box 7, which seems rater unlikely.
Still, it's odd that the plan administrator would mark box 2b Taxable amount not determined. if the distribution was from an employer-provided plan. Unless the administrator lost your plan-participation records, the administrator should know the taxable amount, should not have marked that box, should have shown the taxable amount in box 2a and should have shown the nontaxable portion in box 5. It's implausible that the distribution from a designated Roth account would be entirely taxable.
With the code B in box 7, I suspect that in addition to marking that box cox 2a is blank and I suspect that as a result TurboTax is treating the entire distribution as taxable.
You should contact the payer to determine what really should be on this Form 1099-R.
