youtube-dl: 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 CentOS possible default full paths are:

/usr/bin/youtube-dl - from package: youtube-dl

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/youtube-dl - from package: youtube-dl

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

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