If Box 2a of your 1099-R is blank, it means the taxable portion of your distribution hasn't been determined. You should leave it blank when entering the information into TurboTax. As you continue through the questions, TurboTax will ask you some questions regarding your non-deductible contributions and then calculate the taxable amount based on your information.
For more detailed information about Box 2a and what it means, you can check out this TurboTax article