I mailed in my tax return in Sept. after filing an extension to the address stated in the form. Why did I get a letter saying that there is no record of a tax return?

Address on return form:
Department of the Treasury
Internal Revenue Service
Fresno, Ca 93888

letter stated: On November 11, 2020, we received a request for verification of non-filing of
a tax return.
As of the date of this letter, we have no record of a processed tax return
for the tax period listed above.