RESTSHARP errors ( IRestResponse, Method.GET, Execute)

Question

public virtual string GetPaymentStatus(HttpContext context, NGeniusConfiguration configuration)
        {
            var reference = context.Request.Query[NGeniusConstants.ApiQueryStringRef].ToString();

            var token = GetAccessToken(configuration);
            var url = new Uri(GetPaymentBaseUrl(configuration), string.Format(NGeniusConstants.ApiUrl.OrderStatus, configuration.OutletId, reference));

            var client = new RestClient(url);
            var request = new RestRequest(Method.GET);
            request.AddHeader("authorization", string.Format(NGeniusConstants.ApiauthorizationMode.Bearer, token));
            request.AddHeader("accept", NGeniusConstants.ApiPaymentAcceptJson);
            IRestResponse response =client.Execute(request);

            if (response.StatusCode == HttpStatusCode.OK)
            {
                var status = JsonConvert.DeserializeObject<NGeniusOrderStatusResponse>(response.Content);

                return status.Embedded.Payment.First().State;
            }
            else
            {
                throw new Exception("Error while getting the order status", response.ErrorException);
            }
        }

I am fairly new to .Net Core. Can someone tell me why I have errors in Method.GET, IRestResponse and Execute? I have already included "using RestSharp;" in the program as well. for more reference the the project i am implementing is from GITHUB.