ASP.NET Web API - The requested resource does not support http method 'GET'

I am make POST call into my Web API using Postman. I get the error: "The requested resource does not support http method 'GET'." I am not making a GET call. If I do call one of my GET methods, it works fine and returns the expected result.

My controller class:

[RoutePrefix("api/login")]
    public class LoginController : ApiController
    {
        ModelContext db = new ModelContext();

        [HttpPost]
        [Route("validate")]
        public HttpResponseMessage Validate([FromBody] LoginViewModel login)
        {
            try
            {

                    var message = Request.CreateResponse(HttpStatusCode.OK);
                    return message;
            }
            catch (Exception ex)
            {
                var message = Request.CreateErrorResponse(HttpStatusCode.BadRequest, ex.Message);
                return message;
            }
        }        

    }

I have the Web API running locally and call with this URL:

http://localhost:44303/api/login/validate

This url returns:

<Error>
<Message>
The requested resource does not support http method 'GET'.
</Message>
</Error>

My routing in WebApiConfig.cs

config.MapHttpAttributeRoutes();

            config.Routes.MapHttpRoute(
                name: "DefaultApi",
                routeTemplate: "api/{controller}/{id}",
                defaults: new { id = RouteParameter.Optional }
            );

While this controller message returns HttpResponseMessage, I have tested by changing the response to "string" and just returning a string value, but I get the same error. I have read through so many SO posts but none appear to fix my issue. I am at a total loss to explain this behavior. I appreciate any ideas.

EDIT I have tested GETs in other controllers and they are returning data as expected.

EDIT FOR CONTEXT 6/3/2020 This method in the default ValuesController works:

[Route("api/values")]
public IEnumerable<string> Get()
{
   return new string[] { "value1", "value2" };
}

These 2 methods in the SAME controller do not work:

// GET api/values/5
public string Get(int id)
{
  return "value";
}

// POST api/values
public void Post([FromBody]string value)
{
}

EDIT #2 6/3/2020 Now all of my api methods are working for the default ValuesController. I do not know why. My custom POST methods in other controllers such as the Post method above are still not working. Here is my current WebApiConfig.cs:

public static void Register(HttpConfiguration config)
        {
            // Web API configuration and services

            // Web API routes
            config.MapHttpAttributeRoutes();

            config.Routes.MapHttpRoute(
                name: "Api_Get",
                routeTemplate: "api/{controller}/{action}/{id}",
                defaults: new { id = RouteParameter.Optional, action = "Get" },
                constraints: new { httpMethod = new HttpMethodConstraint(HttpMethod.Get) }
            );

            config.Routes.MapHttpRoute(
                name: "Api_Post",
                routeTemplate: "api/{controller}/{action}/{id}",
                defaults: new { id = RouteParameter.Optional, action = "Post" },
                constraints: new { httpMethod = new HttpMethodConstraint(HttpMethod.Post) }
            );
        }

I do not have any middleware or special routing that I know of. Any exception handling would be simple try-catch.

I am trying to use attribute routing vs convention but that seems to be an issue.