mkisofs: 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/mkisofs - from package: genisoimage

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

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