mprev: 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/mprev - from package: mpdtoys

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

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