Why am I getting charged an Underpayment penalty when I paid my taxes before the deadline? Because when I run the TurboTax program it said there were no errors.

See this IRS info

https://www.irs.gov/businesses/small-businesses-self-employed/estimated-taxes#:~:text=If%20you%20don't%20pay,you%20file%20your%20tax%20return.

You must pay your taxes in a timely manner. 

• This means the IRS expects to be paid each quarter for the amount that you earned in that quarter. 
• The IRS default is that you earned an equal amount each quarter.
• TurboTax will provide you with 1040-ES form to make your estimated payments.
• If you fail to do so, you will accumulate a penalty.
• The penalty is calculated on form 2210 .
• This will show each quarter and the penalty and interest that has accumulated.

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