ping6: 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:

/bin/ping6 - from package: iputils
/usr/sbin/ping6 - from package: iputils

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

On Debian possible default full paths are:

/bin/ping6 - from package: inetutils-ping
/bin/ping6 - from package: iputils-ping

You may install the required package with command apt-get install {package}

Error in other languages:
ping6: bevel nie gevind nie
ping6: comandă negăsită
ping6: comando não encontrado
ping6: commande introuvable
ping6: command not found
ping6: käsku ei ole
ping6: không tìm thấy lệnh
ping6: komanda nerasta
ping6: Kommando nicht gefunden.
ping6: kommandot finns inte
ping6: komut yok
ping6: nie znaleziono polecenia
ping6: níor aimsíodh an t-ordú
ping6: no se encontró la orden
ping6: no s'ha trobat l'ordre
ping6: opdracht niet gevonden
ping6: parancs nem található
ping6: perintah tidak ditemukan
ping6: príkaz nenájdený
ping6: příkaz nenalezen
ping6: команда не найдена
ping6: командата не е открита
ping6: 命令找不到
ping6: コマンドが見つかりません