Change the Model classes to below:

public class Data {

@SerializedName("gender")
@Expose
private String gender;
@SerializedName("weight")
@Expose
private Double weight;
@SerializedName("height")
@Expose
private Double height;
@SerializedName("goals")
@Expose
private String goals;
@SerializedName("activity")
@Expose
private String activity;
@SerializedName("age")
@Expose
private Integer age;
@SerializedName("bmi")
@Expose
private Double bmi;
@SerializedName("condition")
@Expose
private Integer condition;

public String getGender() {
return gender;
}

public void setGender(String gender) {
this.gender = gender;
}

public Double getWeight() {
return weight;
}

public void setWeight(Double weight) {
this.weight = weight;
}

public Double getHeight() {
return height;
}

public void setHeight(Double height) {
this.height = height;
}

public String getGoals() {
return goals;
}

public void setGoals(String goals) {
this.goals = goals;
}

public String getActivity() {
return activity;
}

public void setActivity(String activity) {
this.activity = activity;
}

public Integer getAge() {
return age;
}

public void setAge(Integer age) {
this.age = age;
}

public Double getBmi() {
return bmi;
}

public void setBmi(Double bmi) {
this.bmi = bmi;
}

public Integer getCondition() {
return condition;
}

public void setCondition(Integer condition) {
this.condition = condition;
}

}

UserStatsResponse.java

public class UserStatsResponse {

@SerializedName("sucess")
@Expose
private Boolean sucess;
@SerializedName("status")
@Expose
private Integer status;
@SerializedName("data")
@Expose
private Data data;

public Boolean getSucess() {
return sucess;
}

public void setSucess(Boolean sucess) {
this.sucess = sucess;
}

public Integer getStatus() {
return status;
}

public void setStatus(Integer status) {
this.status = status;
}

public Data getData() {
return data;
}

public void setData(Data data) {
this.data = data;
}

}

First,Using JsonObject or String receive the response.

Check it can has correct response?
I guess the problem what happend in your java bean serialize.

Do you forget config your retrofit support gson convert?

eg:.addConverterFactory(GsonConverterFactory.create(getGson()))

I think problem is here.

if(userStatsResponse.isSuccess ()){
                        Log.d("error", String.valueOf ( userStatsResponse ) );
                        Intent intent = new Intent ( UserStatistics.this, DashboardActivity.class );
                        intent.setFlags ( Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK );
                        startActivity ( intent );
                    }

In userStatsResponse class u have isSucess() method not isSuccess().
try replacing

isSuccess()

in if condition with

isSucess()

While integrating an API using retrofit in an Android project, I am getting a null response from the API when making a post request therefore I am not able to save any data in the shared preferences, I am not able to find the error in logcat when i am able to see that the response is null.

Activity

 Call<UserStatsResponse>  call = RetrofitClient.getInstance ().getApi ().userInfo ( token,gender,weight,height,goals,activity,age );
    call.enqueue ( new Callback<UserStatsResponse> () {
        @Override
        public void onResponse(Call<UserStatsResponse> call, Response<UserStatsResponse> response) {
            UserStatsResponse userStatsResponse = response.body ();

                if(userStatsResponse.isSucess ()){
                    Log.d("error", String.valueOf ( userStatsResponse ) );
                    Intent intent = new Intent ( UserStatistics.this, DashboardActivity.class );
                    intent.setFlags ( Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK );
                    startActivity ( intent );

                }
                else{
                    Toast.makeText ( UserStatistics.this, "Some error", Toast.LENGTH_SHORT ).show ();
                }


                }

        @Override
        public void onFailure(Call<UserStatsResponse> call, Throwable t) {
            Toast.makeText ( UserStatistics.this, t.getMessage (), Toast.LENGTH_SHORT ).show ();

        }
    } );

Data (model class)

public class Data {
private String gender;
private float weight;
private float height;
private String goals;
private String activity;
private int age;
private double bmi;
private int condition;

public Data(String gender, float weight, float height, String goals, String activity, int age, double bmi, int condition) {
    this.gender = gender;
    this.weight = weight;
    this.height = height;
    this.goals = goals;
    this.activity = activity;
    this.age = age;
    this.bmi = bmi;
    this.condition = condition;
}

public String getGender() {
    return gender;
}

public float getWeight() {
    return weight;
}

public float getHeight() {
    return height;
}

public String getGoals() {
    return goals;
}

public String getActivity() {
    return activity;
}

UserStatsResponse

public class UserStatsResponse {
private boolean sucess;
private int status;
private Data data;

public UserStatsResponse(boolean sucess, int status, Data data) {
    this.sucess = sucess;
    this.status = status;
    this.data = data;
}

public boolean isSucess() {
    return sucess;
}

public int getStatus() {
    return status;
}

public Data getData() {
    return data;
}
}

Post Request

@POST("info/profile/")
Call<UserStatsResponse> userInfo(
        @Header ( "Authorization" ) String token,
        @Field ( "gender" ) String gender,
        @Field("weight") int weight,
        @Field("height") int height,
        @Field ( "goals" ) String goals,
        @Field ( "activity" ) String activity,
        @Field ( "age" ) int age

);

JSON Response

{
"sucess": true,
"status": 200,
"data": {
    "gender": "2",
    "weight": 51.0,
    "height": 148.0,
    "goals": "4",
    "activity": "2",
    "age": 21,
    "bmi": 23.283418553688826,
    "condition": 2
}}

//logcat error before and after changing the model classes, the problem still persists

2020-06-06 20:07:29.453 15964-15964/com.example.fitnessapp E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.fitnessapp, PID: 15964
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Boolean com.example.fitnessapp.model_class.UserStatsResponse.getSucess()' on a null object reference
    at com.example.fitnessapp.activities.UserStatistics$1.onResponse(UserStatistics.java:151)
    at retrofit2.ExecutorCallAdapterFactory$ExecutorCallbackCall$1$1.run(ExecutorCallAdapterFactory.java:70)
    at android.os.Handler.handleCallback(Handler.java:883)
    at android.os.Handler.dispatchMessage(Handler.java:100)
    at android.os.Looper.loop(Looper.java:214)
    at android.app.ActivityThread.main(ActivityThread.java:7682)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:516)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)

Retrofit Client

public class RetrofitClient {
private static final String BASE_URL = "https://kanishkarrevin.pythonanywhere.com/";
private static RetrofitClient mInstanceSignUp;
private Retrofit retrofitSignUp;
private RetrofitClient(){
    retrofitSignUp = new Retrofit.Builder()
            .baseUrl(BASE_URL)
            .addConverterFactory( GsonConverterFactory.create())
            .build();
}


public static synchronized RetrofitClient getInstance(){
    if(mInstanceSignUp == null){
        mInstanceSignUp = new RetrofitClient();

    }
    return mInstanceSignUp;
}
public Api getApi(){
    return  retrofitSignUp.create(Api.class);
}

}