PPTP is supported only on older iPads / iPhones running iOS versions up to iOS 9.
Starting with iOS 10 Apple discontinued PPTP support because they consider it outdated and insecure.
Consider using PPTP only when none of the other VPN protocols (IKEv2, L2TP and OpenVPN) are working for you.
On your iPad home screen tap the Settings icon. In General settings tap VPN
(General / Network / VPN on older devices).
2. Tap Add VPN Configuration...