mrtg-ping-probe: 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/bin/mrtg-ping-probe - from package: mrtg-ping-probe

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