monfailures: 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/monfailures - from package: mon

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

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