pppdump: 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/pppdump - from package: ppp

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

On Debian possible default full paths are:

/usr/sbin/pppdump - from package: ppp

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

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