Need help linking a textview to firebase using android studio

I have been trying to display the user data saved on the firebase database to display on my app via TextView. I am not sure where my error is occurring or maybe my read/write permissions are wrong

    FirebaseAuth mAuth;
    FirebaseDatabase mFirebaseDatabase;
    TextView username;
    DatabaseReference myRef;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_profile_settings);
        mAuth = FirebaseAuth.getInstance();
        mFirebaseDatabase = FirebaseDatabase.getInstance();
        myRef = mFirebaseDatabase.getReference().child("users");
        FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
        String userID = user.getUid();
        username = (TextView) findViewById(R.id.username);

        DatabaseReference ref = FirebaseDatabase.getInstance().getReference("uid");
        ref.child(userID).addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                String fName = dataSnapshot.child("fName").getValue().toString();
                username.setText(fName);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {
                Toast.makeText(getApplicationContext(),"Profile information failed.",Toast.LENGTH_LONG).show();
            }
        });

Here is my firebase rules

service cloud.firestore {
  match /databases/{database}/documents {
    match /{document=**} {
      allow read, write: if request.auth == null;
    }
  }
}