mpsyt: 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/mpsyt - from package: mps-youtube

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

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