Why is 1040 Line 20b not "0." for dual-citizen Canadian resident on US SSI.?

If you override anything in Turbotax, you void the accuracy guarantee and you can not efile.

SSI with a SSI-1099; as per IRS Publication 915, line 20b should be "0." in my case (as outlined above). Turbotax correctly recognizes me as a Canadian resident and even reminds me to submit a "FBAR" (as required) . . . but then miscalculates line 20b

Is this SSI, or SSDI? SSI is not taxable at all; SSDI is potentially taxable like all other Social Security. Do you have an SSA-1099 from Social Security?

There is no "SSI-1099"; as I said before, SSI is never taxable. Is it an SSA-1099?

Look . . . your right SSA-1099 . . . SSI is never taxable in the above situation as per the USA/Canada tax treaty . . . I'm just hoping to efile with Turbotax but the problem is iterated in the very first comment above. How about a solution to that issue?

You seem to imply that you have access to a "Turbotax employee" . . . great! . . . maybe that will produce an answer. As for your other comments: 1) See Publication 915, Section entitled, "Are Any of Your Benefits Taxable?", Subsection entitled, "U.S. citizens residing abroad" and find . . .
U.S. citizens who are residents of the following countries are exempt from U.S. tax on their benefits.
    Canada.
    Egypt.
    Germany.
    Ireland.
    Israel.
    Italy. (You must also be a citizen of Italy for the exemption to apply.)
    Romania.
    United Kingdom.
  The SSA will not withhold U.S. tax from your benefits if you are a U.S. citizen.
2) I am a retired person; I am not disabled and do not receive SSDI.

You have SSDI, not SSI. Still, you have a tax treaty situation that I'll have to ask a TurboTax employee to look into. (Publication 915 does confirm that Social Security is not taxable for U.S. citizens residing in Canada.)

Further to the comment above, Publication 915 goes on to instruct the taxpayer (and Turbotax?) as to how the tax form should be completed (isn't this what Turbotax is all about?) in the situation outlined above in the Subsection entitled, "Benefits not taxable." as follows . . .
" If you are filing Form 1040EZ, do not report any benefits on your tax return. If you are filing Form 1040 or Form 1040A, report your net benefits (the total amount from box 5 of all your Forms SSA-1099 and Forms RRB-1099) on Form 1040, line 20a; or Form 1040A, line 14a. Enter -0- on Form 1040, line 20b;"

Hence, the initial question herein above.

I don't work for Intuit, but I do have an "Ask TurboTax to answer" button. Since this is a potential programming issue, I clicked that button to get them. (Edit: I just did it again.)

It doesn't matter whether it's regular Social Security or SSDI; the point is it is *NOT* "SSI" (Supplemental Security Income) as you kept saying. SSI, a form of welfare, is never taxable. Social Security, whether regular or SSDI, *may* be taxable except for the Canadian tax treaty issue you raised. The issue is how to get TurboTax to recognize that the tax treaty applies.

Are you a living as a Resident in Canada, and therefore report and pay Canadian tax on the US Social Security?

If so, yes, you will probably need to override, in which case you will not be able to e-file.  Other than manually overriding things, TurboTax doesn't do anything for Tax Treaty provisions.