Unable to use retrofit2 convertor

So I'm trying to use retrofit GsonConvertorFactory but in the android studio, it is not reading because of which I'm unable to run.

    public class start {

            private  static final String id = "XXXXXXXXXXXXXXXXXXXXXXXXXX";
            private  static final String url = "https://XXX/YY/ZZ";

            public static PostService postService = null;
            public static PostService getService()

            {
                if (postService == null) {
                    Retrofit retrofit = new Retrofit.Builder().baseUrl(url).addConverterFactory(GsonConvertorFactory.create()).build();
                    postService = retrofit.create((PostService.class));
                }
                return postService;
            }


        public interface PostService{
            @GET("?=id"+id )
            Call<PostDia> getPostDia();
        }
    }