PPTP VPNs no longer supported by Apple’s built-in VPN client on macOS Sierra and iOS 10
Starting in OS X El Capitan and iOS 9, people trying to set up a PPTP VPN connection on their iOS device or on their Mac would get a warning that looked like this:
The reason for these warnings is that a number of security vulnerabilities have been found in this VPN communications protocol. These warnings have been Apple’s way of encouraging their customers to stop using PPTP for their VPN connections and move on to other more secure VPN protocols.
As part of preparing for the release of macOS Sierra and iOS 10, Apple has publicly announced they’re moving from warning folks about PPTP to removing PPTP support altogether from Apple’s built-in VPN client. In place of PPTP, Apple is again recommending the use of other VPN communications protocols that are more secure.
For those who will still need to access PPTP VPNs, you may be able to use a third-party client to do so on macOS Sierra. A couple of third-party VPN clients I’m aware of which currently support PPTP on OS X El Capitan are Shimo and VPN Tracker.