rtmon: 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/rtmon - from package: iproute

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

On Debian possible default full paths are:

/sbin/rtmon - from package: iproute2

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

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