mkinitrd: 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:

/sbin/mkinitrd - from package: dracut

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

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