diffmon: 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/diffmon - from package: diffmon

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

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