It sounds as if when you reached the normal retirement age,
your disability payments were transferred to the Social Security
Administration.
I can't find where this is documented for OPM disability recipients, but it
does happen for other retirement systems where the treatment of your income
changes when you reach the "normal" retirement age.
If indeed you are supposed to receive Social Security, then enter the SSA-1099
into TurboTax in the appropriate place - go to Federal Taxes->Wages &
Income->Retirement Plans and Social Security and click on "Social
Security (SSA-1099, RRB-1099)".
However, it may be prudent to call the contact number on the letter you
received to confirm that it is correct that your disability benefit was
transferred to Social Security. If there is no contact information, you might
try looking at page 2 of this brochure "Information
for Disability Annuitants", written for CSRS retirees, for their
contact information.
