My only income is Social Security - Why won't the system let me e-file?
The system most likely is not letting you e-file because you don’t owe any taxes. TurboTax will ask you to enter all your sources of income; if your Social Security Benefits are your only source of income, you most likely do not owe any taxes and are not required to file a tax return.
However, if you still want to file a return (even though you aren’t required to), you can file by mail. See How do I file a paper (mailed) return in TurboTax Online?