monthly: 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/sbin/monthly - from package: wwwstat

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

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