Solved
I contributed $1200.00 to my 401(k) in 2016.
If this is a 401k at work, the answer is because it has already been excluded from your income. That's one reason why the amounts on your W-2 in Blocks 1 and 3 are the same. Having been excluded from taxable wages in Box 1, you can't deduct it again.
Enter your E-mail address. We'll send you an e-mail with instructions to reset your password.