postfix-to-mailman.py: 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/lib/mailman/bin/postfix-to-mailman.py - from package: mailman

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

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