What happened to box 2a taxable amount

Please clarify. Are you saying that Box 2a is empty, or says UNKNOWN, on your 1099-R?  If so, leave that box blank and answer the follow up questions.

Make sure you are entering your 1099-R information in the correct section of TurboTax..

 

The input would be under the Federal income section as follows: 

1. Select Income & Expenses 
2. Scroll down to All Income 
3. Select show more to the right of Retirement Plans & Social Security 
4. Select start/revisit to the right of IRA, 401(k), Pension Plan Withdrawals (1099-R) 
5. To enter additional 1099-R forms, simply select +Add Another 1099-R

Once you get to the input section, your input screen will be similar to the screen below.  Box 2a will be listed and you can input your amount there.