Modal form - cant set error message, and no message for required field
I know i'm a bit late but the problem is that the email field is required.
Required fields may not be empty, otherwise it will throw an error.

If you wish to show an error message you need a form element to catch these messages.

 $form['status_messages'] = [
      '#type' =&gt; 'status_messages',
      '#weight' =&gt; -10,
    ];