pw2userdb: 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/pw2userdb - from package: courier-authlib-userdb

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

Error in other languages:
pw2userdb: bevel nie gevind nie
pw2userdb: comandă negăsită
pw2userdb: comando não encontrado
pw2userdb: commande introuvable
pw2userdb: command not found
pw2userdb: käsku ei ole
pw2userdb: không tìm thấy lệnh
pw2userdb: komanda nerasta
pw2userdb: Kommando nicht gefunden.
pw2userdb: kommandot finns inte
pw2userdb: komut yok
pw2userdb: nie znaleziono polecenia
pw2userdb: níor aimsíodh an t-ordú
pw2userdb: no se encontró la orden
pw2userdb: no s'ha trobat l'ordre
pw2userdb: opdracht niet gevonden
pw2userdb: parancs nem található
pw2userdb: perintah tidak ditemukan
pw2userdb: príkaz nenájdený
pw2userdb: příkaz nenalezen
pw2userdb: команда не найдена
pw2userdb: командата не е открита
pw2userdb: 命令找不到
pw2userdb: コマンドが見つかりません