pppoe-discovery: 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/sbin/pppoe-discovery - from package: ppp

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

On Debian possible default full paths are:

/usr/sbin/pppoe-discovery - from package: ppp

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

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