If you used TurboTax you will have an email telling you when your return was sent and processed.  If you did not send the return or if it was not accepted, then send it again and make sure it goes through.

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.

When the IRS refers to a December 2020 return in a letter, they may just be simply referring to a return for the tax period ending December 31, 2020;  i.e, as opposed to the rare individual who might use a fiscal year ending in some other month.    So a "December 2020 return" may simply be identifying it as the usual 2020 return for the tax period ending 12/31/2020. 

At least that's my guess.   My transcript, for example, says it is for "tax period December 31, 2020."

@ chrissy-clayton3 

Is the IRS saying they have no record that they have received a 2020 return from you?   What are they asking you to do--are they asking you to file a 2020 return?

If you did already file a 2020 return, how did you submit your 2020 return--did you efile, or did you file a paper return?   If you filed by mail, did you physically print, sign, and mail it?  TurboTax does not mail it for you.

If you efiled, check the status this way:

If you used Online TurboTax for your 2020 return, log into your account and at the Tax Home page, what is shown for the status-- which of these terms:  accepted, rejected, printed, started, ready to mail, or what?