Android cannot find xml items

I'm creating an android app with a button, and my xml looks like this:

<Button
    android:id="@+id/start"
    android:text="Start"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" />

Then I want to get the Button in my main class, but I can't.

Button start;
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main_menu);
    start = findViewById(R.id.start);
}

But when I do this, the application won't run because of an error with this line:

start = findViewById(R.id.start);

But I most definitely identified the Button. Can anyone explain why this isn't working?