postfix-add-filter: 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/postfix-add-filter - from package: postfix

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

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