This happens because you have defined a Theme in your styles.xml that has something like:
<style name="AppTheme" parent="Theme.AppCompat.Light.**NoActionBar**">
You can manually create a toolbar, and then yo have to add it to your Activity.
The xml for the toolbar view could be something like:
<?xml version="1.0" encoding="utf-8"?>
<androidx.appcompat.widget.Toolbar
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_height="wrap_content"
android:layout_width="match_parent"
android:minHeight="?attr/actionBarSize"
android:background="?attr/colorPrimary"
android:elevation="4dp"
android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
app:popupTheme="@style/ThemeOverlay.AppCompat.Light">
</androidx.appcompat.widget.Toolbar>
Then, in the Activity, you have to "display" it :
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
ActionBar actionBar = getSupportActionBar();
if (actionBar != null) {
actionBar.setDisplayShowTitleEnabled(false);
}
And now, you can create the menu with the known onCreateOptionsMenu