What do i need to do if box 2a states unknown

Are you sure that you're not looking at your 1099-R form for your pension?  

Your SSA-1099 form for your Social Security benefits does not have a Box 2a;  Box 2 on your SSA-1099 form lists your Social Security Number.

When you enter the 1099-R in the program, if box 2a says UNKNOWN leave it blank in TurboTax. 

Don't enter anything.  Later on, when it asks you if the amount in box 2a was used as the taxable amount in past years, answer Yes, even though box 2a is blank.  The program will use the amount in box 1 as the taxable amount.

Add/Edit/Delete Form 1099-R (Distributions From Pensions, Annuities, Retirement or Profit-Sharing Plans, IRAs, Insurance Contracts, etc.):

1. Open your return (if not already open).
2. Enter "1099r" in the Search box.
3. Click on "jump to 1099r" link.  
4. Enter you information manually.

Add/Edit/Delete Form SSA-1099 (Social Security Benefit Statement):

1. Open your return (if not already open).
2. Enter "ssa1099" in the Search box.
3. Click on "jump to ssa1099" link.  
4. Enter you information manually.