Service methods cannot return void. retrofit

Question

This is my method in Interface. I am calling this function but app crash with this exception:

Caused by: java.lang.IllegalArgumentException: Service methods cannot return void. for method RestInterface.getOtp

//post method to get otp for login
@FormUrlEncoded
@POST("/store_login")
void getOtp(@Header("YOUR_APIKEY") String apikey, @Header("YOUR_VERSION") String appversion,
            @Header("YOUR_VERSION") String confiver, @Field("mobile") String number, Callback<Model> cb);

And this is the code where I am calling this function

Retrofit retrofit = new Retrofit.Builder()
            .baseUrl(API_URL)
            .build();

    RestInterface restApi = retrofit.create(RestInterface.class);
    restApi.getOtp("andapikey", "1.0", "1.0", "45545845454", new Callback<Model>() {

        @Override
        public void onResponse(Response<Model> response) {

        }

        @Override
        public void onFailure(Throwable t) {

        }
    });

Answer 1

There is difference in Asynchronous in Retrofit 1.9 and 2.0

/* Synchronous in Retrofit 1.9 */

public interface APIService {

@POST("/list")
Repo loadRepo();

}

/* Asynchronous in Retrofit 1.9 */

public interface APIService {

@POST("/list")
void loadRepo(Callback<Repo> cb);

}

But on Retrofit 2.0, it is far more simple since you can declare with only just a single pattern

/* Retrofit 2.0 */

public interface APIService {

@POST("/list")
Call<Repo> loadRepo();

}

// Synchronous Call in Retrofit 2.0

Call<Repo> call = service.loadRepo();
Repo repo = call.execute();

// Asynchronous Call in Retrofit 2.0

Call<Repo> call = service.loadRepo();
call.enqueue(new Callback<Repo>() {
@Override
public void onResponse(Response<Repo> response) {

   Log.d("CallBack", " response is " + response);
}

@Override
public void onFailure(Throwable t) {

  Log.d("CallBack", " Throwable is " +t);
}
});