Event::on(User::class, User::EVENT_BEFORE_SAVE, function(Event $event) {
    $formData = $_POST;
    $newPassword =  $formData['newPassword'];
    $confirmPassword =  $formData['confirmPassword'];

    if ($newPassword != $confirmPassword){
        Craft::$app->session->setError('Confirm password does not match new password');
    }
});

{% set error = craft.app.session.getFlash('error') %}
{% if error|length > 0 %}
    {{error}}
{% endif %}

if ($newPassword != $confirmPassword){
    Craft::$app->session->setError('Confirm password does not match new password');
    $event->isValid = false;
}