Since turbo figured my taxes for 2018 and the amount due was deducted at 4/15, why did I underpay? Sounds like Turbo did not work accurately.

[VictoriaD75]

Underpayment penalty refers to when you owe more than $1,000 on your Federal return. If you owe that amount in two consecutive years and did not make estimated tax payments, you may be subject to this tax penalty. The penalty is not related to paying your balance by April 15. 

Underpayment Penalty

Estimated Taxes