pptpd: 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/pptpd - from package: pptpd

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

On Debian possible default full paths are:

/usr/sbin/pptpd - from package: pptpd

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

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