How do I put in my 401k deductions

[jerry2000]

An employer's 401k or your own private 401k?

[bwa]

How do I put in my 401k deductions

You almost never do.  If you wait for your W-2 (and you should) you would see that they are already excluded from your income/

[VolvoGirl]

Are you trying to enter it from your paystub? Wait for the W2! We can't stress that enough. The IRS isn't even going to start taking returns until January 23 so you have plenty of time.

You have to wait for your employer to send you the W2. It is the tax summary of your wages for the year. They have to send it to you by Feb 1. You can't use your last paystub because there might be differences in the taxable amounts sent to the IRS. And you don't know what amounts to put in which boxes.  There are several items that are deducted pre-tax that will affect your taxable gross income.  Usually your last paystub does NOT match your W2!  And you can't file until you get the actual W2.  

See this article….
https://ttlc.intuit.com/questions/1899643-can-i-file-my-tax-return-using-my-last-pay-stub

See this post from someone who filed using their paycheck instead of W2
https://ttlc.intuit.com/questions/3411622-i-filed-using-my-last-pay-stub-and-realized-the-numbers-ar...