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?
