clientupdate: 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/lib/xymon/client/bin/clientupdate - from package: xymon-client

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

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