thcping6: 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/thcping6 - from package: thc-ipv6

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

Error in other languages:
thcping6: bevel nie gevind nie
thcping6: comandă negăsită
thcping6: comando não encontrado
thcping6: commande introuvable
thcping6: command not found
thcping6: käsku ei ole
thcping6: không tìm thấy lệnh
thcping6: komanda nerasta
thcping6: Kommando nicht gefunden.
thcping6: kommandot finns inte
thcping6: komut yok
thcping6: nie znaleziono polecenia
thcping6: níor aimsíodh an t-ordú
thcping6: no se encontró la orden
thcping6: no s'ha trobat l'ordre
thcping6: opdracht niet gevonden
thcping6: parancs nem található
thcping6: perintah tidak ditemukan
thcping6: príkaz nenájdený
thcping6: příkaz nenalezen
thcping6: команда не найдена
thcping6: командата не е открита
thcping6: 命令找不到
thcping6: コマンドが見つかりません