mpd-rate: 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/mpd-rate - from package: libaudio-mpd-perl

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

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