TurboTax isn't stating an underpayment on your 2019 return.
It's using last year's tax liability as a baseline for the amount of tax you'd have needed to pay in 2020. That's what's triggering the underpayment penalty.
You must have a balance due on your 2020 return of $500 or more. If that's the case, your withholdings and/or estimated payments this year must not have equaled or exceeded 100% of your 2019 tax liability.
According to LINE 33 (Estimated tax penalty) in the Kansas 2020 Individual Income Tax Booklet:
An estimated tax penalty may be due if the total of your withholding and estimated tax payments (lines 22 and 23) subtracted from line 19 is $500 or more. Complete Schedule K-210 to determine the penalty amount to enter on line 33. There are two exceptions: 1) if withholdings and/or estimated payments (lines 22 and 23) equal or exceed 100% of the prior year’s tax liability (line 18 from last year’s return) or, 2) if your withholdings and/or estimated payments (lines 22 and 23) equal or exceed 90% of this year’s total income tax (line 19).
