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

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

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