Can't save data in RTDB

With this code I'm supposed to be able to retrieve a value directly from a RTDB (realtime database) and show it in the screen of my app, but nothing happens, app compiles fine. I hope you could help me.

On the image lays the simple structure of my RTDB

public class MainActivity extends AppCompatActivity {

private static final  String PATH_START = "start";
private static final  String PATH_MESSAGE = "message";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    final TextView tvMessage = findViewById(R.id.tvMessage);

    FirebaseDatabase database = FirebaseDatabase.getInstance();
    final DatabaseReference reference = database.getReference(PATH_START)
            .child(PATH_MESSAGE);



    reference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot snapshot) {
               tvMessage.setText(snapshot.getValue(String.class));
        }

        @Override
        public void onCancelled(@NonNull DatabaseError error) {
            Toast.makeText(MainActivity.this, "not able to read in Firebase",
                    Toast.LENGTH_LONG).show();

        }
    });


  }
}