The “XamlCTask” task failed unexpectedly Error

I'm currently using Xamarin.forms and i'm trying to build but i keep getting this error :

Error MSB4018: The "XamlCTask" task failed unexpectedly.
System.IO.FileNotFoundException: Could not load file or assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies. Le fichier spécifié est introuvable.
File name: 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'
at Xamarin.Forms.Build.Tasks.XamlCTask.Execute(IList`1& thrownExceptions)
at Xamarin.Forms.Build.Tasks.XamlTask.Execute()
at Microsoft.Build.Framework.ITask.Execute()
at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
at Microsoft.Build.BackEnd.TaskBuilder.d__26.MoveNext()

I already saw some posts but any of them is resolving this problem and i can't start working because of this issue . Any help .. ? Thank you .