mrtg: 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 CentOS possible default full paths are:

/usr/bin/mrtg - from package: mrtg

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/mrtg - from package: mrtg

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

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