Does the letter have an IRS  number on it (like, LTR 6714 or something like that)?  My guess is that they have one form letter that is used for both 1120 and 1120-S, and they didn't make all the changes.  It's sometimes easier to research, with that letter number. 

You should answer, or the account for 2019 and future years might be frozen for lack of response.  You should not fax your response, because of the problems reported earlier this year with IRS fax machines.  They ran out of ink, and the technicians couldn't get in the building to service them.