ipppd: 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/ipppd - from package: isdn4k-utils

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

On Debian possible default full paths are:

/usr/sbin/ipppd - from package: ipppd

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

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