How to solve the below error, I'm getting this error when i'm building the game for windows phone 8.

Error building Player: Exception: Error: method System.Boolean System.Threading.WaitHandle::WaitOne(System.Int32,System.Boolean) doesn't exist in target framework. It is referenced from Assembly-CSharp.dll at System.Void MegaModifiers::DoWork(System.Object). Error: method System.String System.Environment::GetEnvironmentVariable(System.String) doesn't exist in target framework. It is referenced from Assembly-CSharp.dll at System.Void MegaGrab::DoGrabTGA(). Error: method System.String System.Environment::GetEnvironmentVariable(System.String) doesn't exist in target framework. It is referenced from Assembly-CSharp.dll at System.Void MegaGrab::DoGrabJPG(). Error: method System.Boolean System.Threading.WaitHandle::WaitOne(System.Int32,System.Boolean) doesn't exist in target framework. It is referenced from Assembly-CSharp.dll at System.Void MegaBookBuilder::DoWork(System.Object). Error: method System.Void System.IO.File::AppendAllText(System.String,System.String) doesn't exist in target framework. It is referenced from Assembly-CSharp.dll at System.Void CSVReader::SaveScoreToCsv(Score). Error: method System.String[] System.IO.File::ReadAllLines(System.String) It is referenced from Assembly-CSharp.dll at System.Void CSVReader::ReadDescp(System.String). Error: method System.String[] System.IO.File::ReadAllLines(System.String) doesn't exist in target framework. It is referenced from Assembly-CSharp.dll at System.Void CSVReader::ReadGeneralSetting().