xymoncmd: 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/xymoncmd - from package: xymon-client
/usr/lib/xymon/client/bin/xymoncmd - from package: xymon-client
/usr/lib/xymon/server/bin/xymoncmd - from package: xymon

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

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