Why “Send to messenger” plugin doesn’t show?

I have the "send to messenger" plugin that supposed to show but doesn’t show. If this helps you in any way that would be supreme.

My page:

    window.fbAsyncInit = function () {
            xfbml: true,
            version: 'v9.0'

        FB.Event.subscribe('send_to_messenger', function (e) {

    (function (d, s, id) {
        var js, fjs = d.getElementsByTagName(s)[0];
        if (d.getElementById(id)) return;
        js = d.createElement(s); js.id = id;
        js.src = 'https://connect.facebook.net/en_US/sdk/xfbml.customerchat.js';
        fjs.parentNode.insertBefore(js, fjs);
    }(document, 'script', 'facebook-jssdk'));

<div class="fb-send-to-messenger" messenger_app_id="XXXX" page_id="XXXX"  
    data-ref="anything" color="blue" size="large">

I added my webhook to the whitelist, as well as, I allowed the cookies.

I used everything in production (Heroku PasS), but the same, the button not showing.

There is something I missed? Thanks in advance

Source: Facebook SDK Questions