Applicable ProductsiOS 10 and later

Root CauseThis is a known issue caused by changes introduced in iOS 10 that removed support for built-in L2TP/PPTP connections.

SolutionTo continue using VPN connections, Apple recommends using a more secure VPN protocol.

L2TP/IPSecIKEv2/IPSecCisco IPSecAdditionally, you may try to install and use an SSL VPN client from the Apple App Store.

Further ReadingPrepare for removal of PPTP VPN before you upgrade to iOS 10 and macOS Sierra