Why I can't create a fragment by fragment tag in Android

Question

I'm in trouble with fragment tag as you can see below:

• My app contain 2 buttons: Fragment 1, Fragment 2
• Also 2 fragment: FirstFragment.java and SecondFragment.java
• Then I set 1 fragment tag which I use as a container for 1 of 2 fragments

Question:

• I used getSupportFragmentManager() to set FirstFragment.java into my fragment tag but it doesn't work. Why?
• If I change fragment tag into FrameLayout, FragmentContainerView it will work. Why?
• How to using fragment tag as a container and calling it in onCreate() programmatically?

My code:

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        FirstFragment firstFragment = new FirstFragment();
        SecondFragment secondFragment = new SecondFragment();
        getSupportFragmentManager().beginTransaction().add(R.id.flFragment, firstFragment).commit();
        findViewById(R.id.btnFrag1).setOnClickListener(view -> {
            getSupportFragmentManager().beginTransaction().replace(R.id.flFragment, firstFragment).commit();
        });
        findViewById(R.id.btnFrag2).setOnClickListener(view -> {
            getSupportFragmentManager().beginTransaction().replace(R.id.flFragment, secondFragment).commit();
        });
    }
}

    <Button
        android:id="@+id/btnFrag1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Fragment 1"
        app:layout_constraintEnd_toStartOf="@+id/btnFrag2"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/btnFrag2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Fragment 2"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toEndOf="@+id/btnFrag1"
        app:layout_constraintTop_toTopOf="parent" />

    <fragment
        android:id="@+id/flFragment"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/btnFrag1" />