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

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

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