nut-monitor: 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/nut-monitor - from package: nut-client

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

On Debian possible default full paths are:

/usr/bin/nut-monitor - from package: nut-monitor

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

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