My firebase Realtime database path is not working

I am making an app based on firebase Realtime database.

I want my database structure like the tree given below:-

I am trying to get that data written in my firebase Realtime database by the following java code:-

public class AddActivity extends AppCompatActivity {

    private EditText etName;
    EditText etRoll;
    Button btnAdmit;
    Button btnView;

    String stName;
    String stRoll;
    FirebaseDatabase myfire;
    DatabaseReference myRef;

    @Override
    public void onBackPressed() {

    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_add);
        etName = (EditText) findViewById(R.id.idUser);
        etRoll = (EditText) findViewById(R.id.idPass);
        btnView = (Button) findViewById(R.id.idView);

        btnAdmit= (Button) findViewById(R.id.idAdmit);
        myfire = FirebaseDatabase.getInstance();

        myRef = myfire.getReference("Users")
                            .child("uid");// I dont know what
                                          // to write in this child
                                          // in stead of
                                          // "uid"
       

        btnAdmit.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                stName = etName.getText().toString();
                stRoll = etRoll.getText().toString();


                etName.setText("");
                etRoll.setText("");



                myRef.child("101").addValueEventListener(new ValueEventListener() {

                    @Override
                    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                       model model =new basic();
                       model.setFb01name(stName);
                       model.setFb04roll(stRoll);
                       myRef.setValue(model);
                       LayoutInflater inflater = getLayoutInflater();
                       View layout = inflater.inflate(R.layout.toast,
                                (ViewGroup) findViewById(R.id.toast_id));
                       TextView text = (TextView) layout.findViewById(R.id.idToast);
                        text.setText("Added.");
                        Toast toast = new Toast(AddActivity.this);
                        toast.setGravity(Gravity.CENTER ,0, 0);
                        toast.setDuration(Toast.LENGTH_SHORT);
                        toast.setView(layout);
                        toast.show();
                       // btnAdmit.setEnabled(false);
                       // btnAdmit.setVisibility(View.GONE);
                    }




                    @Override
                    public void onCancelled(@NonNull DatabaseError databaseError) {
                        LayoutInflater inflater = getLayoutInflater();
                        View layout = inflater.inflate(R.layout.toast,
                                (ViewGroup) findViewById(R.id.toast_id));
                        TextView text = (TextView) layout.findViewById(R.id.idToast);
                        text.setText(" Database Error.");
                        Toast toast = new Toast(getApplicationContext());
                        toast.setGravity(Gravity.CENTER ,0, 0);
                        toast.setDuration(Toast.LENGTH_LONG);
                        toast.setView(layout);
                        toast.show();

                        DatabaseError e = null;

                        Log.e("Database", "Error", e.toException());
                    }



                });
            }
        });

My model class goes like this:

public class model {

        String fb01name;
        String fb04roll;

        public model() {

        }

        public model(String fb01name, String fb04roll) {
                this.fb01name = fb01name;
                this.fb04roll = fb04roll;
        }



        public String getFb01name() {
                return fb01name;
        }

        public void setFb01name(String fb01name) {
                this.fb01name = fb01name;
        }

        public String getFb04roll() {
                return fb04roll;
        }

        public void setFb04roll(String fb04roll) {
                this.fb04roll = fb04roll;
        }
}

There is nothing in logcat error message. I am getting nothing written in the firebase Realtime database. I am not able to find the error. The app is displaying the toast "added" as given in on data change method. My database rules are like this:

{
// Allow anyone to read data, but only authenticated content owners can
// make changes to their data

  "rules": {
    "Users": {
      "$uid": {
        ".read": true,
        // or ".read": "auth.uid != null" for only authenticated users
        ".write": "auth.uid == $uid"
      }
    }
  }
}

Please look into the matter and provide your insight and practical solution.