Can I deduct what I contribute to 401k plan? Deductions screen asking how much I contributed to traditional IRA in 2018.

[BMcCalpin]

If you contribute to your employer's 401(k) plan, that is shown as code D in box 12 on your W-2. This amount is removed from Wages in box 1 (you can usually see this by comparing box 1 to box 3).

So in this case, you already have the deduction because it was removed from your reported income.

If you are asking if you can have your own 401(k) plan, the answer is "yes, sort of". If you are self-employed, then you can have a One-Participant 401(k) Plan (see https://www.irs.gov/retirement-plans/one-participant-401k-plans).

Otherwise, you need to contribute to some sort of IRA.