pptp-portslave: 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/pptp-portslave - from package: pptpd

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

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