mrtg-traffic-sum: 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-traffic-sum - from package: mrtg

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

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