xtables-multi: 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:

/sbin/xtables-multi - from package: iptables

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

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