ipv6.mpb: 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"

Error in other languages:
ipv6.mpb: bevel nie gevind nie
ipv6.mpb: comandă negăsită
ipv6.mpb: comando não encontrado
ipv6.mpb: commande introuvable
ipv6.mpb: command not found
ipv6.mpb: käsku ei ole
ipv6.mpb: không tìm thấy lệnh
ipv6.mpb: komanda nerasta
ipv6.mpb: Kommando nicht gefunden.
ipv6.mpb: kommandot finns inte
ipv6.mpb: komut yok
ipv6.mpb: nie znaleziono polecenia
ipv6.mpb: níor aimsíodh an t-ordú
ipv6.mpb: no se encontró la orden
ipv6.mpb: no s'ha trobat l'ordre
ipv6.mpb: opdracht niet gevonden
ipv6.mpb: parancs nem található
ipv6.mpb: perintah tidak ditemukan
ipv6.mpb: príkaz nenájdený
ipv6.mpb: příkaz nenalezen
ipv6.mpb: команда не найдена
ipv6.mpb: командата не е открита
ipv6.mpb: 命令找不到
ipv6.mpb: コマンドが見つかりません