If your only income was Social Security, then you are not required to file a tax return. If line 11 is zero on your Form 1040, you cannot e-file. If you just really want to e-file it anyway for some reason, then enter a fake $1 of interest, which is a trick the IRS allows so that you can e-file.
Who has to file?
http://www.irs.gov/uac/Do-I-Need-to-File-a-Tax-Return%3F
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