error referencia.net 4.6.1 for O. netStandart 2.0

I'm having a problem referencing a Standart2. 0 project to my .NET Framework 461 api;

Occurs the following I have the Class:

public class MeuReturn : IHttpActionResult
{
        private readonly string _objeto;

        public MeuReturn(string objeto)
        {
            _objeto = objeto;
        }
        public Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken)
        {
            HttpResponseMessage response = 
                                        new HttpResponseMessage(HttpStatusCode.BadRequest)
            {
                Content = new StringContent(_objeto)
            };
            return Task.FromResult(response);
        }

}

To make a custom return, it works 100% but when I reference the other project and squeegee the API the following error occurs:

An error occurred. Method ExecuteAsync in Type WebApplication3.Controllers.Assembly MeuReturn WebApplication3, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null it does not have an implementation. System.TypeLoadException on WebApplication3.Controllers.HomeController.error() in lambda_method (Closure , Object , Object[] ) in System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor. c _ _ DisplayClass10.b _ _ 9 (Object instance, Object [] methodParameters) in System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.Execute (Object instance, Object [] arguments) em System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync (HttpControllerContext controllerContext, IDictionary ' 2 arguments, CancellationToken cancellationToken) - - - end of previous location stack trace where the exception was raised - - - in System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (Task task) in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (Task task) in System.Web.Http.Controllers.ApiControllerActionInvoker.d _ _ 0.MoveNext() --- End of stack trace from previous location where exception was raised - - - in System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (Task task) in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (Task task) in System.Web.Http.Controllers.ActionFilterResult.d _ _ 2.MoveNext() --- End of stack trace from previous location where exception was raised - - - in System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (Task task) in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (Task task) in System.Web.Http.Dispatcher.HttpControllerDispatcher.d _ _ 1.MoveNext()

I checked again the Microsoft website and theoretically are compatible