mpinsert: 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/mpinsert - from package: mpdtoys

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

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