pppoeconf: 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 Debian possible default full paths are:

/usr/sbin/pppoeconf - from package: pppoeconf

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

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