mrtg-apache: 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/mrtg-apache - from package: mrtgutils

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

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