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

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