If your 1099-R has UNKNOWN as the taxable amount, you probably have a CSA 1099-R.
Be sure to pick the correct 1099-R type: Standard 1099-R, CSA-1099-R, CSF-1099-R, RRB-1099-R.
After you enter the 1099R in the screen, turbo tax will ask additional questions about your 1099R to determine how much of that distribution is taxable.
Either way, if Box 2a reads unknown, leave it blank and TurboTax will provide you with follow-up questions.