userdbpw: 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/userdbpw - from package: courier-authlib-userdb

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

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