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

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

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