Hi @rebeccat1844660 - I don't think the File Extraction activity will output a header-only CSV file. You have to have at least one data row for it to output the header. Strangely it will create a blank file if there are no rows.
I would use a simple JS execCommand activity to create your header-only CSV e.g.
execCommand("echo header_1,header_2,header_3 > /sftp/your/path/goes/here/output_file.csv");
Cheers
Darren