mudpasswd: 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/bin/mudpasswd - from package: muddleftpd

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

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