SimpleJson get 502 error

I do not understand what is wrong. I set up a connection to my server and registered endpoints, but when I request data I get an error 502

namespace Grafana
{
    [ApiController]
    public class BugsMonitoringController : ControllerBase
    {
        
        [HttpGet]
        [Route("/")]
        public void Empty()
        {
        }

        [HttpPost]
        [Route("query")]
        public HttpResponseMessage Query()
        {
            return new HttpResponseMessage()
            {
                Content = new StringContent("[{\"columns\":[{\"text\":\"Time\",\"type\":\"time\"},{\"text\":\"Country\",\"type\":\"string\"},{\"text\":\"Number\",\"type\":\"number\"}],\"rows\":[[1234567,\"SE\",182],[1234567,\"DE\",282],[1234567,\"US\",382]],\"type\":\"table\"}]", Encoding.UTF8, "application/json")
            };
        }

        [HttpPost]
        [Route("search")]
        public HttpResponseMessage Search()
        {
            return new HttpResponseMessage()
            {
                Content = new StringContent("[ { \"text\" :\"upper_25\", \"value\": 1}, { \"text\" :\"upper_75\", \"value\": 2} ]", Encoding.UTF8, "application/json")
            };            
        }

        [HttpPost]
        [Route("annotations")]
        public HttpResponseMessage Annotation()
        {
            return new HttpResponseMessage()
            {
                Content = new StringContent("[{\"text\":\"text shown in body\",\"title\":\"Annotation Title\",\"isRegion\":true,\"time\":\"timestamp\",\"timeEnd\":\"timestamp\",\"tags\":[\"tag1\"]}]", Encoding.UTF8, "application/json")
            };            
        }
        
    }
}

When I send POST request in Postman (maybe this will help to understand the problem):