In doing this year's taxes, I see a notification that I owe $70 for "underpayment of estimated taxes", however I paid the total tax due on the 15th of April. ??

On April 15 of 2019, you would have paid taxes for your 2018 return. During 2019 you were expected to pay estimated taxes due to your underpayment in 2018. If you underpaid again in 2019, that is what you are seeing.

Tax Topic 306

In doing this year's taxes,

So that would be your 2019 tax return.

I see a notification that I owe $70 for "underpayment of estimated taxes",

Which would indicate that you did not pay enough taxes each quarter of 2019, or if a W-2 employee, you did not have enough withheld from each of your paychecks by your employer.

I paid the total tax due on the 15th of April. ??

So if you did not pay your 4th quarter 2019 taxes by Jan 15th of 2020, that forth quarter payment was late.

If, at the time you file your taxes, what you owe the IRS is more than $1000 or more than 10% of your total tax liability (whichever is *HIGHER*) then an underpayment penalty will be assessed.

Take a look at your 1040. If line 19 is less than line 16 and if the difference is more than $1000 or more than 10% (whichever is HIGHER) then you underpaid your quarterly taxes during the year and a penalty is assessed. The penalty is on line 24, and like that line is labeled, it's an "estimated" penalty. The penalty stops accruing on the date the IRS receives your payment; not the date you file your tax return. So if it's more the IRS will send you a physically separate bill in the U.S. Mail for the additional amount.