youtube-download: 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/youtube-download - from package: libwww-youtube-download-perl

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

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