When you pay your tax in 2021 does not impact the underpayment penalty amount. When you don't have enough tax withholding and you don't make any estimated tax payments during the year, then the IRS or your state can charge you with an underpayment penalty
There are some exceptions to this penalty that you may qualify for if you meet any of these requirements
You paid in 90% of your 2020 tax liability (90% of line 24)
You paid in 100% of your 2019 tax liability during 2020 (or 110% if your AGI is over $150,00)
To visit the section where you can go through possible exceptions to the penalty follow these steps
Login to your return and click "Take Me To My Return"
Click "Search" in the upper right corner of your screen, then type "underpayment penalty" in the Search Box and click "Find"
Click the "Jump To" Link
From here, you can review the underpayment penalty and work through the possible exceptions
If you do not pay you tax by 5/17, then the IRS can charge a failure to pay penalty in addition to the underpayment penalty
