You have 2 paths to take in the program ... the easiest is if you do not know for sure that some or all of the distribution is non taxable that makes it all taxable so just enter the same amount in box 2a from box 1 and your problems are over.
That is probably a CSA 1099-R from OPM.
OPM violates the 1099-R rules by writing "helpful" information in the box rather than just the value or code like they are supposed to. OPM has done that for years and it causes more confusion than help. They put UNKNOWN in box 2a when it should be blank, and put "7 NONDISABILITY" in box 7 when is should only be "7", "1" or "2".
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".
Enter a 1099-R here:
Wages & Income
I’ll choose what I work on (if that screen comes up),
Retirement Plans & Social Security,
IRA, 401(k), Pension Plan Withdrawals (1099-R).
OR Use the "Tools" menu (if online version under My Account) and then "Search Topics" for "1099-R" which will take you to the same place.
Be sure to choose which spouse the 1099-R is for if this is a joint tax return.
Be sure to pick the correct 1099-R type: Standard 1099-R, CSA-1099-R, CSF-1099-R, RRB-1099-R.
[NOTE: When you get to the "Your 1099-R Entries" screen where you can add another 1099-R, use "continue" to keep going as there are additional interview questions after that screen in most cases. You can always return as shown above.]