yum: 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/yum - from package: yum

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

On Debian possible default full paths are:

/usr/bin/yum - from package: yum

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

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