Could not load file or assembly Xamarin.Forms.Core, Version=1.2.1.0

Hi, guys! I'am need help. Сompiler issues a message:
+ $exception {System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.IO.FileLoadException: Could not load file or assembly 'Xamarin.Forms.Core, Version=1.2.1.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)
at CalendarMaster.WinPhone.MainPage..ctor()
--- End of inner exception stack trace ---
at System.Windows.Navigation.PageResourceContentLoader.EndLoad(IAsyncResult asyncResult)
at System.Windows.Navigation.NavigationService.ContentLoader_BeginLoad_Callback(IAsyncResult result)} System.Exception {System.Reflection.TargetInvocationException}