upsmon: 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/sbin/upsmon - from package: nut-client

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

On Debian possible default full paths are:

/sbin/upsmon - from package: nut-client

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

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