FTL is not listening on localhost:4711, so the web interface can not connect to it. Check the logs at /var/log/pihole-FTL.log and journalctl -u pihole-FTL -l for more information on why that might be happening.
[2019-01-15 19:58:33.243] ########## FTL started! ##########
[2019-01-15 19:58:33.243] FTL branch: master
[2019-01-15 19:58:33.243] FTL version: v4.1.2
[2019-01-15 19:58:33.243] FTL commit: b06eedf
[2019-01-15 19:58:33.243] FTL date: 2018-12-21 14:43:34 -0600
[2019-01-15 19:58:33.243] FTL user: pihole
[2019-01-15 19:58:33.243] Starting config file parsing (/etc/pihole/pihole-FTL.conf)
[2019-01-15 19:58:33.243] SOCKET_LISTENING: only local
[2019-01-15 19:58:33.243] AAAA_QUERY_ANALYSIS: Show AAAA queries
[2019-01-15 19:58:33.243] MAXDBDAYS: max age for stored queries is 365 days
[2019-01-15 19:58:33.243] RESOLVE_IPV6: Resolve IPv6 addresses
[2019-01-15 19:58:33.243] RESOLVE_IPV4: Resolve IPv4 addresses
[2019-01-15 19:58:33.243] DBINTERVAL: saving to DB file every minute
[2019-01-15 19:58:33.243] DBFILE: Using /etc/pihole/pihole-FTL.db
[2019-01-15 19:58:33.243] MAXLOGAGE: Importing up to 24.0 hours of log data
[2019-01-15 19:58:33.243] PRIVACYLEVEL: Set to 0
[2019-01-15 19:58:33.243] IGNORE_LOCALHOST: Show queries from localhost
[2019-01-15 19:58:33.243] BLOCKINGMODE: Null IPs for blocked domains
[2019-01-15 19:58:33.243] REGEX_DEBUGMODE: Inactive
[2019-01-15 19:58:33.243] ANALYZE_ONLY_A_AND_AAAA: Disabled. Analyzing all queries
[2019-01-15 19:58:33.243] DBIMPORT: Importing history from database
[2019-01-15 19:58:33.243] PIDFILE: Using /var/run/pihole-FTL.pid
[2019-01-15 19:58:33.243] PORTFILE: Using /var/run/pihole-FTL.port
[2019-01-15 19:58:33.243] SOCKETFILE: Using /var/run/pihole/FTL.sock
[2019-01-15 19:58:33.243] WHITELISTFILE: Using /etc/pihole/whitelist.txt
[2019-01-15 19:58:33.243] BLACKLISTFILE: Using /etc/pihole/black.list
[2019-01-15 19:58:33.243] GRAVITYFILE: Using /etc/pihole/gravity.list
[2019-01-15 19:58:33.243] REGEXLISTFILE: Using /etc/pihole/regex.list
[2019-01-15 19:58:33.244] SETUPVARSFILE: Using /etc/pihole/setupVars.conf
[2019-01-15 19:58:33.244] AUDITLISTFILE: Using /etc/pihole/auditlog.list
[2019-01-15 19:58:33.244] Finished config file parsing
[2019-01-15 19:58:33.244] Compiled 0 Regex filters and 60 whitelisted domains in 0.0 msec (0 errors)
[2019-01-15 19:58:33.244] Database successfully initialized
[2019-01-15 19:58:33.244] Imported 0 queries from the long-term database
[2019-01-15 19:58:33.244] -> Total DNS queries: 0
[2019-01-15 19:58:33.244] -> Cached DNS queries: 0
[2019-01-15 19:58:33.244] -> Forwarded DNS queries: 0
[2019-01-15 19:58:33.244] -> Exactly blocked DNS queries: 0
[2019-01-15 19:58:33.244] -> Unknown DNS queries: 0
[2019-01-15 19:58:33.244] -> Unique domains: 0
[2019-01-15 19:58:33.244] -> Unique clients: 0
[2019-01-15 19:58:33.244] -> Known forward destinations: 0
[2019-01-15 19:58:33.244] Successfully accessed setupVars.conf
journalctl -u pihole-FTL -l
Jan 15 19:58:33 networking-host systemd[1]: Starting LSB: pihole-FTL daemon...
Jan 15 19:58:33 networking-host pihole-FTL[3639]: Not running
Jan 15 19:58:33 networking-host su[3667]: Successful su for pihole by root
Jan 15 19:58:33 networking-host su[3667]: + ??? root:pihole
Jan 15 19:58:33 networking-host su[3667]: pam_unix(su:session): session opened for user pihole by (uid=0)
Jan 15 19:58:33 networking-host pihole-FTL[3639]: dnsmasq: failed to create listening socket for port 53: Address already in use
Jan 15 19:58:33 networking-host systemd[1]: Started LSB: pihole-FTL daemon.