fragmentation6: 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/fragmentation6 - from package: thc-ipv6

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

Error in other languages:
fragmentation6: bevel nie gevind nie
fragmentation6: comandă negăsită
fragmentation6: comando não encontrado
fragmentation6: commande introuvable
fragmentation6: command not found
fragmentation6: käsku ei ole
fragmentation6: không tìm thấy lệnh
fragmentation6: komanda nerasta
fragmentation6: Kommando nicht gefunden.
fragmentation6: kommandot finns inte
fragmentation6: komut yok
fragmentation6: nie znaleziono polecenia
fragmentation6: níor aimsíodh an t-ordú
fragmentation6: no se encontró la orden
fragmentation6: no s'ha trobat l'ordre
fragmentation6: opdracht niet gevonden
fragmentation6: parancs nem található
fragmentation6: perintah tidak ditemukan
fragmentation6: príkaz nenájdený
fragmentation6: příkaz nenalezen
fragmentation6: команда не найдена
fragmentation6: командата не е открита
fragmentation6: 命令找不到
fragmentation6: コマンドが見つかりません