fuzz_ip6: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/fuzz_ip6 - from package: thc-ipv6

You may install the required package with command yum install {package}

Error in other languages:
fuzz_ip6: bevel nie gevind nie
fuzz_ip6: comandă negăsită
fuzz_ip6: comando não encontrado
fuzz_ip6: commande introuvable
fuzz_ip6: command not found
fuzz_ip6: käsku ei ole
fuzz_ip6: không tìm thấy lệnh
fuzz_ip6: komanda nerasta
fuzz_ip6: Kommando nicht gefunden.
fuzz_ip6: kommandot finns inte
fuzz_ip6: komut yok
fuzz_ip6: nie znaleziono polecenia
fuzz_ip6: níor aimsíodh an t-ordú
fuzz_ip6: no se encontró la orden
fuzz_ip6: no s'ha trobat l'ordre
fuzz_ip6: opdracht niet gevonden
fuzz_ip6: parancs nem található
fuzz_ip6: perintah tidak ditemukan
fuzz_ip6: príkaz nenájdený
fuzz_ip6: příkaz nenalezen
fuzz_ip6: команда не найдена
fuzz_ip6: командата не е открита
fuzz_ip6: 命令找不到
fuzz_ip6: コマンドが見つかりません