I am already contributing more to my 401K in 2020 because I'm working overtime and I only started saving at the end of 2019. It won't let my 2020 through 4/15 be larger

[DoninGA]

Contributions to an employer 401(k) tax deferred retirement account can only be contributed to during the calendar year of the plan.  Any contributions made in 2020 will be for the 2020 plan year.

[VolvoGirl]

And you only enter the 401K from your W2, nowhere else.  Do not enter it under Deductions and a contribution.  A 401K is not an IRA.