yumdownloader: 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/yumdownloader - from package: yum-utils

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

On Debian possible default full paths are:

/usr/bin/yumdownloader - from package: yum-utils

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

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