nmon: 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 Debian possible default full paths are:

/usr/bin/nmon - from package: nmon

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

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