First, it would be reported in either box 12a, 12b, 12c, or 12d. All four boxes for line 12 on the W-2 are the same - what matters is the code next to the dollar amount: DD, D, W, C, etc. The code indicates what the dollar amount refers to, not the a, b, c, or d.
Second, when a contribution to a SIMPLE retirement account is done by income deferral (i.e., salary reduction), then, yes, box 12 (any one of the boxes) has an S and the dollar amount removed from income and sent to the SIMPLE account by the employer.
Third, you are correct, if you have an S in box 12 of your W-2, then you don't enter this elsewhere in TurboTax as a contribution to a traditional IRA. As the Learn More in TurboTax says, "A traditional IRA is any IRA that is not a Roth IRA or a SIMPLE IRA." Generally, in TurboTax, you enter each number just once, and once done, you don't need to enter it anywhere else.
As for entering zero in the Traditional Contribution field, I don't know why you can't. What are you trying to do?
@mcad006
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