icmp6: 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 Debian possible default full paths are:

/usr/sbin/icmp6 - from package: ipv6toolkit

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

Error in other languages:
icmp6: bevel nie gevind nie
icmp6: comandă negăsită
icmp6: comando não encontrado
icmp6: commande introuvable
icmp6: command not found
icmp6: käsku ei ole
icmp6: không tìm thấy lệnh
icmp6: komanda nerasta
icmp6: Kommando nicht gefunden.
icmp6: kommandot finns inte
icmp6: komut yok
icmp6: nie znaleziono polecenia
icmp6: níor aimsíodh an t-ordú
icmp6: no se encontró la orden
icmp6: no s'ha trobat l'ordre
icmp6: opdracht niet gevonden
icmp6: parancs nem található
icmp6: perintah tidak ditemukan
icmp6: príkaz nenájdený
icmp6: příkaz nenalezen
icmp6: команда не найдена
icmp6: командата не е открита
icmp6: 命令找不到
icmp6: コマンドが見つかりません