What is the possible error for this retrofit code? I can't understand the connection of my API and MOBILE

You almost reached to the solution. But, you made little mistake while passing parameters to the API request.

As I can see from the screenshot of Insomia app, that API requires JSONArray as parameter but you're sending JSONObject.

Sample JSON parameter

[    {        "SerialNumber" : "1234",        "Coordinate1" : "12.7845874",        "Coordinate2" : "76.4584578",        "DateTime" : "2018-11-14 08:45:00",        "Speed" : "0",        "Port" : 9090    }]

According to the above JSON structure you need to change the Api.java class to something like this:

import retrofit2.Call;import retrofit2.http.Body;import retrofit2.http.POST;import java.util.List;  // add importpublic interface Api {    @POST("/api/Database/NewLocation")    Call < MapDetails > mapDetailLocation(@Body List<MapDetails> mapDetails);                                            //  ^^^^ changes here    @POST("/api/Registration/RegisterDevice")    Call < RegisterDetails > registerDetails(@Body RegisterDetails registerAllDetails);}

Add List<MapDetails> to mapDetailLocation() method parameter.

And in Activity or Fragment use above method like this:

//......// part of the codeMapDetails mapDetails = new MapDetails("1807200005", lat, lon, currentDateTimeString, "0", 9090);List<MapDetails> data = new ArrayList<>();data.add(mapDetails);Retrofit.Builder builder = new Retrofit.Builder()                            .baseUrl("<BASE_URL>")  // change base URL                            .addConverterFactory(GsonConverterFactory.create());Retrofit retrofit = builder.build();Api locate = retrofit.create(Api.class);Call<MapDetails> call = locate.mapDetailLocation(data);     // NOTICE THE CHANGES IN PARAMETERcall.enqueue(new Callback<MapDetails>() {    @Override    public void onResponse(Call<MapDetails> call, Response<MapDetails> response) {        // do whatever you want    }    @Override    public void onFailure(Call call, Throwable t) {        // log the error message     }});

Note: Please change base URL according to your requirement.

Edit:

Change method parameters in Activity from MapDetails to List<MapDetails>

// prepare dataMapDetails data = new MapDetails("1807200005", lat, lon, currentDateTimeString, "0", 9090);// add it to ArrayListList<MapDetails> mapDetails = new ArrayList<>();mapDetails.add(data);// pass it as an argumentprivate void setLocation(List<MapDetails> mapDetails) {    initializeRetrofit(mapDetails);}

Change method parameter in initializeRetrofit()

private void initializeRetrofit(List<MapDetails> mapDetails) {    Retrofit.Builder builder = new Retrofit.Builder()                            .baseUrl("<BASE_URL>")  // change base URL                            .addConverterFactory(GsonConverterFactory.create());    Retrofit retrofit = builder.build();    Api locate = retrofit.create(Api.class);    SetMapLocationApiCaller(locate, mapDetails);}

Again change method parameter

private void SetMapLocationApiCaller(Api locate, List<MapDetails> mapDetails) {    Call<MapDetails> call = locate.mapDetailLocation(mapDetails);    executeCallAsynchronously(call);}