xenmon: command not found

Have You tried accessing the binary with the full path?

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/lib/xen-4.4/bin/xenmon - from package: xen-utils-4.4

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