netfilter-persistent: 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/netfilter-persistent - from package: netfilter-persistent

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

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