the error message indicates the files are on your server and should be available to you via ftp. (it’s been much longer ago that there was a “simple” option that used inaccessible files.)
Navigate in panel to “manage domains” and check the “USER” that appears in the “web hosting” column for the domain in question. (this appears to be user “fnseal” from the error message, but double check on manage domains.) Make sure you are using that user when logging in. From your home directory click in to the directory named the same as your domain name, and there you will find your files.