Your model is not representing same data as in JSON. With UserOrtu class it will works only if your response will have structure like below:

{
  "id":1,
  "nama": "Name",
  "email":"email",
  "message":"msg"
}

But as you can see 3 first fields are inside another object data. So your model should look more like:

class LoginResponse{
    @SerializedName("data")
    @Expose
    private UserOrtu userData;
    @SerializedName("message")
    @Expose
    private String message;
}

I want to make a fitur to login, I want to make a feature to log in, initially I saw the response code shows 200, but when I saw the response body was null. I can still log in, but the logged in user data isn’t saved. I used 2 db MySql and SQLite and im using retrofit and shared preference. And here my code

API Interface

public interface Api {
    @FormUrlEncoded
    @POST(Config.API_LOGIN_USER)
    Call<UserOrtu> loginUser(
            @Field("email") String email,
            @Field("password") String password
    );

PrefUtil.java

public class PrefUtil {
    public static final String USER_SESSION = "user_session";
    public static final String USER_STORAGE = "user_storage";

    public static SharedPreferences getSharedPreferences(Context ctx){
        return PreferenceManager.getDefaultSharedPreferences(ctx);
    }

    public static void putUser(Context ctx, String key, UserOrtu user){
        Gson gson = new Gson();
        String json = gson.toJson(user);
        putString(ctx, key, json);
    }

    public static UserOrtu getUser(Context ctx, String key){
        Gson gson = new Gson();
        String json = getString(ctx, key);
        UserOrtu user = gson.fromJson(json, UserOrtu.class);
        return user;
    }

    public static void putString(Context ctx, String key, String value){
        getSharedPreferences(ctx).edit().putString(key, value).apply();
    }

    public static String getString(Context ctx, String key){
        return getSharedPreferences(ctx).getString(key, null);
    }

    public static void clear(Context ctx) {
        getSharedPreferences(ctx).edit().clear().apply();
    }
}

Models
UserOrtu.java

public class UserOrtu {
    @SerializedName("id_user")
    @Expose
    private int idUser;
    @SerializedName("nama")
    @Expose
    private String nama;
    @SerializedName("email")
    @Expose
    private String email;
    @SerializedName("password")
    @Expose
    private String password;
    @SerializedName("error")
    @Expose
    private Boolean error;
    @SerializedName("message")
    @Expose
    private String message;

    public int getIdUser() {
        return idUser;
    }

    public void setIdUser(int idUser) {
        this.idUser = idUser;
    }

    public String getNama() {
        return nama;
    }

    public void setNama(String nama) {
        this.nama = nama;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public Boolean getError() {
        return error;
    }

    public void setError(Boolean error) {
        this.error = error;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

SignInActivity.java

public class SignInActivity extends AppCompatActivity {

    @BindView(R.id.input_email_signin)
    TextInputEditText etEmail;
    @BindView(R.id.text_register)
    TextView tvRegister;
    @BindView(R.id.input_password_signin)
    TextInputEditText etPassword;

    EmailValidator emailValidator;
    PasswordValidator passwordValidator;
    Context context;

    private String email;
    private String password;
    private Api mApi;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (isSessionLogin()){
            startActivity(new Intent(this, MainActivity.class));
            this.finish();
        }

        setContentView(R.layout.activity_sign_in);
        ButterKnife.bind(this);
        AndroidThreeTen.init(this);
        mApi = RetrofitBuilder.builder(this).create(Api.class);

    }

    boolean isEmail(EditText text){
        emailValidator = new EmailValidator();
        String email = text.getText().toString();
        return emailValidator.isValid(email);
    }

    boolean isPassword(EditText text){
        passwordValidator = new PasswordValidator();
        String pass = text.getText().toString();
        return passwordValidator.isValid(pass);
    }

    @OnClick(R.id.text_register) void toRegister(){
        Intent intent = new Intent(this, SignUpActivity.class);
        startActivity (intent);
    }

    @OnClick(R.id.btn_signin) void onLogin(){
        if(isEempty(etEmail)){
            etEmail.setError("Email harus diisi");
        }else if(isEempty(etPassword)){
            etPassword.setError("Password harus diisi");
        }else if(!isEmail(etEmail)){
            etEmail.setError("Email tidak valid");
        }else if(!isPassword(etPassword)){
            String str = passwordValidator.getString();
            Toast.makeText(getApplicationContext(),str, Toast.LENGTH_SHORT).show();
        }else {
            loginAct();
        }
    }
    void loginAct(){
        email = etEmail.getText().toString();
        password = etPassword.getText().toString();
        final MaterialDialog dialog = DialogBuilder.showLoadingDialog(SignInActivity.this, "Updating Data", "Please wait..", false);
        mApi.loginUser(email, password).enqueue(new Callback<UserOrtu>() {
            @Override
            public void onResponse(Call<UserOrtu> call, Response<UserOrtu> response) {
                UserOrtu user = response.body();
                Log.i("USER_LOGIN", response.message());
                if (user != null){
                    //Masih error disini
                    //if (!user.getError()){
                        PrefUtil.putUser(getApplicationContext(), PrefUtil.USER_SESSION, user);
                        Intent intent = new Intent(SignInActivity.this, MainActivity.class);
                        startActivity(intent);
                        //this.finish();

                    //}
                    Toast.makeText(getApplicationContext(), user.getMessage(), Toast.LENGTH_SHORT).show();
                }
                if (response.code() == 403){
                    etPassword.requestFocus();
                    etPassword.setError(getString(R.string.error_password));
                }
                if (response.code() == 404){
                    etEmail.requestFocus();
                    etEmail.setError(getString(R.string.error_login));
                }
                dialog.dismiss();
            }

            @Override
            public void onFailure(Call<UserOrtu> call, Throwable t) {
                //Toast.makeText(getActivity(), t.getMessage(), Toast.LENGTH_SHORT).show();
                dialog.dismiss();
                Log.i("USER_LOGIN", t.getMessage());
                DialogBuilder.showErrorDialog(SignInActivity.this, "Gagal Login");
            }
        });
    }

    // this method to check is user logged in ?
    boolean isSessionLogin(){
        return PrefUtil.getUser(getApplicationContext(), PrefUtil.USER_SESSION) != null;
    }

}

And here’s my result
enter image description here

Here’s my JSON request
enter image description here