flow6: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/flow6 - from package: ipv6toolkit

You may install the required package with command apt-get install {package}
Error in other languages:
flow6: bevel nie gevind nie
flow6: comandă negăsită
flow6: comando não encontrado
flow6: commande introuvable
flow6: command not found
flow6: käsku ei ole
flow6: không tìm thấy lệnh
flow6: komanda nerasta
flow6: Kommando nicht gefunden.
flow6: kommandot finns inte
flow6: komut yok
flow6: nie znaleziono polecenia
flow6: níor aimsíodh an t-ordú
flow6: no se encontró la orden
flow6: no s'ha trobat l'ordre
flow6: opdracht niet gevonden
flow6: parancs nem található
flow6: perintah tidak ditemukan
flow6: príkaz nenájdený
flow6: příkaz nenalezen
flow6: команда не найдена
flow6: командата не е открита
flow6: 命令找不到
flow6: コマンドが見つかりません