I have never paid on 'estimated taxes'. How could I have underpaid? Is this a scam?

[ee-ea]

If at filing time, you have not prepaid enough income taxes through withholding or quarterly estimated payments, you may have to pay a penalty for underpayment. If you have to pay more than $1,000 in taxes, you owe the penalty. If you did not prepay at least 90 percent of the total tax due for this tax year, you owe the penalty. No scam, tax law.

[MinhT1]

Even if you only have wages on a form W-2, you could owe an underpayment penalty if your tax withholdings are insufficient.

You may need to review your form W-4 and resubmit it to your employer to adjust your withholdings.

[DoninGA]

IRS Tax Topic for the Estimated tax penalty - https://www.irs.gov/taxtopics/tc306

The estimated tax penalty is on your federal tax return, Form 1040 Line 38