The error says it's failed on the first chapter, which your posted logs don't list as found in the cache:
"fanficfare.exceptions.HTTPErrorFFF: HTTP Error in FFF 'Page not found or expired in Browser Cache (see FFF setting browser_cache_age_limit)'(428) URL:'https://www.fanfiction.net/s/13051824/1/New-Blood'"
You haven't been skipping step 3 have you? "Navigate in your browser to chapter 1 of the the story you want to download."
https://github.com/JimmXinu/FanFicFa...erCacheFeature