AlarmManager is not showing toast message at specified time

I want my app to run a specific task at exactly 9 pm every day. I have read multiple examples of it being implemented in Android apps, and I have tried almost every one of them, but nothing seems to work. To test the code out, I try to show a toast message at a certain time, but no toast message comes up, even minutes later after the set time because I read on some sites that the Alarm time may not be exact. Please help me figure out what the problem with my code is.

MainActivity.java:

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

        AlarmManager alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
        Intent intent = new Intent(this, MyBroadcastReceiver.class);
        PendingIntent pendingIntent = PendingIntent.getBroadcast(this, 101, intent, PendingIntent.FLAG_UPDATE_CURRENT);
        Calendar calendar = Calendar.getInstance();
        calendar.setTimeInMillis(System.currentTimeMillis());
        calendar.set(Calendar.HOUR_OF_DAY, 21);
        calendar.set(Calendar.MINUTE, 00);
        if (alarmManager != null) {
            alarmManager.setExact(AlarmManager.RTC, calendar.getTimeInMillis(), pendingIntent);
        }
    }

    public class MyBroadcastReceiver extends BroadcastReceiver {
        @Override
        public void onReceive(Context context, Intent intent) {
            Toast.makeText(MainActivity.this, "Alarm...", Toast.LENGTH_SHORT).show();
        }
    }

AndroidManifest.xml:

    <uses-permission android:name="android.permission.WAKE_LOCK" />
    <uses-permission android:name="com.android.alarm.permission.SET_ALARM"/>

Any help is highly appreciated. I have been stuck with this issue for hours. Thank you.