mpv: 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/mpv - from package: mpv

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

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