On your W-2, you have boxes 12a, 12b, 12c, and 12d, all of whom are generically referred to as "box 12".
For each pair of one or two letter codes and dollar amount in boxes 12a-12d, enter this pair in the row when you are your W-2 information in TurboTax (for example, "D" and "$1,000"). At the start, there is only one row for box 12 in TurboTax, but if you have more box 12s on the W-2, then click on "Add another row" just underneath to add another pair, until all the pairs on the W-2 are entered.
If there are no entries in boxes 12a-12d of your W-2, then don't enter anything into TurboTax for row 12.
Please ignore the statement "see instructions for box 12" - this is actually an instruction for your employer who is filling in the fields on the W-2, not for you the taxpayer.