xymon-mailack: 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/server/bin/xymon-mailack - from package: xymon

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

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