Form 3805Q Loss code must be entered

According to 3805Q instructions, the loss code is the sic code for your business, unless it is a disaster loss.  if you had a disaster loss, the codes are listed in the link. You can search these by various web search engines.  Here is a link by the NAICS that may give you the code you are looking for.

I was stuck here too.  I found those 4 digit SIC codes and that seemed to work.  Thanks.