Windows Phone 8 Error

Windows Phone 8 Error: type System.EnvironmentVariableTarget doesn’t exist in target framework. Error: type System.EnvironmentVariableTarget doesn’t exist in target framework

My app works fine in the editor, on iOS and on Android, but I get the following error every time I try to build for the phone:

Error: type System.EnvironmentVariableTarget doesn’t exist in target framework.

I don’t know what this means. It doesn’t even create a Visual Studio project, as it had done with another app that I’m porting to WP8.

Has anyone else seen this error? Any solutions? No hits from my searches in Unity Forums, Answers, or Google.

Just remove the Plugins used for Android and IOs like Prime 31.
then build it.