mdmon: 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/mdmon - from package: mdadm

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

On Debian possible default full paths are:

/sbin/mdmon - from package: mdadm

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

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