monit: 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/monit - from package: monit

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

On Debian possible default full paths are:

/usr/bin/monit - from package: monit

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

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