OPM Form 1099-R has "unknown" in block 2a Taxable Amount.

1099-R's for retirement and pension distributions often have 'Unknown' in Box 2a, Taxable Amount, so it is not unusual. 

Just enter your 1099-R exactly as it is issued, but leave Box 2a blank (don't enter 'unknown' or '0'). 

The taxable amount in box 2a is usually the box 1 amount unless you have after-tax contributions in the retirement plan and use the simplified method. If this is NOT the first year of receiving payments, then you should use the same method that was used last year - either the box 1 amount or the simplified method, using the carry forward simplified information from last year.

If box 2a is blank or UNKNOWN then there should be amount in box 9b to use with the simplified method. If no amount in 9b then contact OPM to find the account "basis".
 

Here's more info on How to Enter CSA-1099-R.