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

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

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