2ping6: 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/2ping6 - from package: 2ping

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

On Debian possible default full paths are:

/usr/bin/2ping6 - from package: 2ping

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

Error in other languages:
2ping6: bevel nie gevind nie
2ping6: comandă negăsită
2ping6: comando não encontrado
2ping6: commande introuvable
2ping6: command not found
2ping6: käsku ei ole
2ping6: không tìm thấy lệnh
2ping6: komanda nerasta
2ping6: Kommando nicht gefunden.
2ping6: kommandot finns inte
2ping6: komut yok
2ping6: nie znaleziono polecenia
2ping6: níor aimsíodh an t-ordú
2ping6: no se encontró la orden
2ping6: no s'ha trobat l'ordre
2ping6: opdracht niet gevonden
2ping6: parancs nem található
2ping6: perintah tidak ditemukan
2ping6: príkaz nenájdený
2ping6: příkaz nenalezen
2ping6: команда не найдена
2ping6: командата не е открита
2ping6: 命令找不到
2ping6: コマンドが見つかりません