kppp: 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/bin/kppp - from package: kdenetwork
/usr/sbin/kppp - from package: kdenetwork

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

On Debian possible default full paths are:

/usr/bin/kppp - from package: kppp

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

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