mdadm: 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/mdadm - from package: mdadm

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

On Debian possible default full paths are:

/sbin/mdadm - from package: mdadm

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

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