The official tax form from TIAA has box 7 with "2B" provided. No value in box 10. 457B is a government employee version of a 401k roth contribution.
The combination of codes 2 and B in box 7 of your Form 1099-R is correct. TurboTax should accept it. If you haven't already, I suggest you go back through the entry of your Form 1099-R and make sure that all entries are correct. If it still won't accept the 2B combination try deleting the form and re-entering. Finally, if it still won't accept it, circle back and post at what point you are seeing an error and what exactly are you seeing, and let us know if you're using TurboTax Desktop or Online.