This is the error occurs in the Client application that is consuming the WEB API. There might be two different reasons,

Web API by default doesn’t allow Service to be consumed from different domains.

  1.   RESOLVED BY CORS:

 [WEB API APPLICATION]

Open Tools ->   NuGet Package Manager - > Package Manager Console 
And install NuGet packages

Install-Package Microsoft.AspNet.WebApi.Cors

     GO to WebApiConfig.cs

          Add [ using System.Web.Http.Cors;  ] namespace in webapiConfig.cs file

               EnableCorsAttribute cors = new EnableCorsAttribute("*", "*", "*");
       CONFIG.EnableCors(cors);


         First star resembles allow requests from all domains, else we can specify specific client URLS, separated by comma (,).

          Second star resembles allow requests for all the headers.

          Third star resembles allow requests for all the methods like POST, GET, PUT, DELETE, etc 


  2.   RESOLVED BY REMOVING HEADERS IN CLIENT REQUEST[AFTER CORS IMPLEMENTATION]

       
While no headers specified in WEB API, just try to remove the headers from the client side.