yumex: 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/yumex - from package: yumex

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

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