ytdl: 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/ytdl - from package: python-pafy

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

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