Why is there a penalty if tax is not due until 4/15?

An estimated tax penalty on the Form 1040 Line 38 is based on the taxes paid in 2023. 

to avoid underpayment penalties these are the rules

There will be no federal penalties for not paying in enough taxes during the year if withholding and

1. timely estimated tax payments equal or exceed 90% of your current year tax or
2. timely estimated tax payments equal or exceed 100% of your prior year's 2022 tax (110% if your prior year's adjusted gross income was more than $150K) or
3. the balance due after subtracting taxes withheld from 90% of your current year's tax is less than $1,000 or
4. your total current year's taxes are less than $1,000

generally, through withholding and/or estimated tax payments, 25% of the required annual tax payments must be paid by the end of each tax period. They are 3/31,5/31,8/31,12/31. this is true for withholding but for estimates, they can be paid by the 15th  of the following month and count as being paid for that period.

uneven payments might require the use of the annualized income installment method on page 3 of the 2210

to reduce or avoid penalties 

state laws differ

Do you have a penalty on 1040 line 38?  It's not for filing or paying late.  It doesn't have to do with last year's tax return or not paying estimates for this year.  The penalty is an "estimated" amount.    It's a penalty if you owe too much or for not paying in enough withholding during the year or not paying in evenly.  Even if you are getting a refund you can still owe a penalty.  It's very common and normal and expected for the IRS or state to bill you for more or send you a refund.

You might be able to eliminate it or at least reduce it.  You can go to Federal Taxes tab or Personal tab, under Other Tax Situations and select Start by the Underpayment Penalties. You will answer a series of questions that may reduce or eliminate the penalty. Or you can elect to have the IRS figure the penalty for you.  It's form 2210.

It's under

Federal or Personal (for Home & Business Desktop)

Other Tax Situations

Additional Tax Payments

Underpayment Penalties - Click the Start or update button