The important thing is NOT box 12a,b,c,d, but the letter code next to the dollar amount. The amount withheld for a 401(k) (probably the most common retirement plan) is marked with letter "D" and a dollar amount (there are other codes for other plans).
In the simplest of cases, you don't need to do anything about this D code outside of entering your W-2.
This box (box 13) is one the W-2 needs nbecause this can be used to calculate the annual contribution limit for an IRA.
To know if it made a difference in your case (because it often or even usually won't), reopen your return (if you have already filed, check that you want to add a state - this will open the return to be amended). Then check the box on your W-2 and see of it makes any differences on your return, like amount due or refund.
If it makes no difference, then just leave it alone and go no farther. Just remember for next year that TurboTax will think that you started the amendment process (you can ignore this).
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