Did you read the second comment?
https://github.com/JimmXinu/FanFicFare/issues/1036#issuecomment-1922143831
https://github.com/JimmXinu/FanFicFare/wiki/SupportedSites
Limited Support Sites
Unfortunately, it has become difficult or impossible to
support some sites
completely, often due to CloudFlare blocking.
For those sites, FanFicFare has implemented Browser Cache and
Proxy
Features to work around the issues.
Notably:
fanfiction.net requires using either the Browser Cache
Feature or Proxy
Features for most users.
https://github.com/JimmXinu/FanFicFare/wiki/BrowserCacheFeature
https://github.com/JimmXinu/FanFicFare/wiki/ProxyFeatures