pppd: 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/pppd - from package: ppp

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

On Debian possible default full paths are:

/usr/sbin/pppd - from package: ppp

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

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