pppoe-relay: 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:

/sbin/pppoe-relay - from package: rp-pppoe
/usr/sbin/pppoe-relay - from package: rp-pppoe

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

On Debian possible default full paths are:

/usr/sbin/pppoe-relay - from package: pppoe

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

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