Ok ... time to track down the missing amount. First look at the return you filed ... form 1040 line 36.
If the amount is on that line then get an IRS account transcript for the 2020 return ... does it show the amount being applied ? https://www.irs.gov/individuals/get-transcript
If so then either call the number on the notice or mail/fax proof of payment (amt applied) from the return and transcript to the address on the notice. Always remember the IRS is not always right and to be polite when dealing with the overworked staff.
