authpasswd: 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/authpasswd - from package: courier-authlib

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

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