did you mail your return? did you have a balance due? if so, when was it paid payment was due on or before 4/15/19)? if not, did you later get a notice saying the iRS changed your return and you now owe.
put another way, even on a timely filed return, a late payment penalty arises when you owe on the return but don't pay by the due date. if you did pay timely, you can contest the penalty by sending back the notice and including proof that the payment was timely. this can be a copy of the cancelled check or if you did direct debit a copy of bank statement or credit card statement.
