The error -1712 is a "timeout" error, meaning that the Application tried to do something, and it couldn't complete the task and so it "timed out". This is usually related to some software conflicting with our Application.
First, please try repairing your disk permissions:
Go to Applications/Utilities
Open Disk Utility
Click Macintosh HD
Click on Verify Disk Permissions (for MacOS 10.11, Click on First Aid at the top)
Click Repair Disk Permissions (for MacOS 10.11, Click Run)
When the check completes, power off your computer, wait five minutes, and turn the computer back on.
After this, please delete the existing copy of the installer you have downloaded, then re-download a fresh copy from our website here, and give the install another try.
If the issue still persists after trying the above, another common cause of this issue is corrupt admin permissions on the current user profile you are using on your computer. The VPN requires admin access to be able to make all the changes necessary to create the VPN connection.
To confirm this is the issue, simply create a new admin profile on the computer in question, and try installing the VPN software on the new profile. If the install works, then this is the case. You will have to contact Apple for assistance with the issues with the admin permissions, or simply import your settings to the new profile and start using it instead.