If your IRS form 1099-R has a value in box 1, no entry in box 2a and code 7, you may have seen the screen We found a few other questions about your situation.
At the screen Any Periodic Payments? did you select Yes, got regular payments from this retirement account?
At the screen Describe the Taxable Amount, did you select No, a different amount was taxable?
At the screen Tell Us the Taxable Amount Method, did you select Simplified method?
Are you reporting an OPM CSA 1099-R or something else?