pptpctrl: 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/pptpctrl - from package: pptpd

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

On Debian possible default full paths are:

/usr/sbin/pptpctrl - from package: pptpd

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

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