It depends on when you withdrew your TSP contributions and the kind of withdrawal you made.
You can make an election under FERCCA if you:
FERCCA does not give you a choice about Social Security coverage. If you should have had Social Security coverage during your Federal employment, then you must have Social Security coverage in addition to your Federal retirement coverage. You have no choice. If your agency incorrectly put you in CSRS when it should have put you in CSRS Offset, it must correct your retirement coverage to CSRS Offset. You will not be able to get the full amount of the refund you were expecting.
Your previous agency should have sent the Social Security Administration a record of your earnings during all the years you should have had Social Security coverage. All of the CSRS contributions you made during those years that are not needed to cover your retirement costs were transferred to Social Security. Your refund was based on the retirement contributions that should have been withheld from your pay. It did not include amounts that were properly withheld, but erroneously considered retirement deductions rather than Social Security taxes.
There was an unexpected error when performing your action.
Your error has been logged and the appropriate people notified. You may close this message and try your command again, perhaps after refreshing the page. If you continue to experience issues, please notify the site administrator.