The FTP will upload what it finds at your standard HTML root
see below examples - the paths will have to be your paths on your weewx local server

while my html root path is /var/weewx/reports, yours might be /var/www/html/weewx
e.g.
[StdReport]
# Where the skins reside, relative to WEEWX_ROOT
SKIN_ROOT = /etc/weewx/skins
# Where the generated reports should go, relative to WEEWX_ROOT
HTML_ROOT = /var/weewx/reports
the html files of the skin which is enabled and has no special HTML root indicated, will be uploaded from this directory
e.g.
[[simple]] # under [StdReport]
skin = simple
# would put only the simple html files there - and that's what the FTP will upload to your web site
if your Seasons skin points somewhere else
e.g.
[[SeasonsReport]] # under [StdReport]
# The SeasonsReport uses the 'Seasons' skin, which contains the
# images, templates and plots for the report.
skin = Seasons
enable = true
HTML_root=/var/weewx/reports/Seasons then the Seasons skin html files will be there inside this directory
for FTP to upload them, you would have to tell FTP where = add the HTML_root information to the FTP entry
e.g.
[[FTP]]
# FTP'ing the results to a webserver is treated as just another report,
# albeit one with an unusual report generator!
skin = Ftp
# If you wish to use FTP, set "enable" to "true", then
# fill out the next four lines.
# Use quotes around passwords to guard against parsing errors.
enable = true
user = username
password = password
server = domainname.tld # The ftp server name, e.g,
www.myserver.org path = /public_html/weewx # The destination directory, e.g., /weather
# Set to True for an FTP over TLS (FTPS) connection. Not all servers
# support this.
secure_ftp = False
# To upload files from something other than what HTML_ROOT is set
# to above, specify a different HTML_ROOT here.
HTML_ROOT = /var/weewx/reports/Seasons # Most FTP servers use port 21
port = 21
# Set to 1 to use passive mode, zero for active mode
passive = 1
#with this blue highlighted entry FTP now knows where to pick up the files it posts to your web site