The late payment or failure-to-pay penalty applies to any portion of your federal tax debt that remains outstanding as of the due date. This is typically the filing date, April 15 in most years. For 2020, this date is July 15.
The IRS imposes a failure-to-pay penalty of 0.5% for each month or part of a month that the tax goes unpaid, up to a total of 25% of the total you owe.