Apple has stopped support for PPTP VPN on MacOS Sierra & iOS 10 as of the September release. While adding many great features (like Siri, Universal Clipboard for cross-device copying and pasting, auto unlock) the out-of-the-box functionality for PPTP VPN was removed. There are solutions available to configure and use your existing PPTP VPN but they all require installing a 3rd party application. The most main stream solution is a 3rd party application called Shimo. The major downside is having to pay $50 because Apple decided to pull support for this VPN type. The free alternative to Shimo is another application called FlowVPN, just be sure to overwrite the default VPN server address when setting up your VPN connection.