Not sure why you are mentioning December. E-filing for 2019 returns closed in October of 2020, so the only way to file a 2019 return after October 15 was by mail. Is December 2020 when you mailed in the return? Does the letter say they did not receive your tax return or does it say that you did not pay your tax due? Your only proof that you paid your tax will be your own bank or credit card statement that shows the payment.
Please explain what you did in December.
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