mplength: 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/mplength - from package: mpdtoys

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

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