mon: 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/mon - from package: mon

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

On Debian possible default full paths are:

/usr/sbin/mon - from package: mon

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

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