When you test your page using Facebook Sharing Debugger and get the warning "The following required properties are missing: fb:app_id",

Magento 2 Required Properties Facebook

you need to add fb:app_id meta tag to the <head> tag of your page:

<meta property="fb:app_id" content="ХХХХХХХХХХХХХХХ" />

Replace ХХХХХХХХХХХХХХХ with your Facebook App ID.

 

If you use Magento 2, you can easily do this without any technical knowledge following the steps bellow:

1. Go to Admin Panel > Content > Design > Configuration and choose the Store View you want the required properties to be added to by pressing the Edit button.

 Magento 2 Required Properties Configuration

2. Once you're there find the HTML Head section and add this code:

<meta property="fb:app_id" content="ХХХХХХХХХХХХХХХ" /> 

to the Scripts and Style Sheets field, example:

Magento 2 Required Properties, HTML Head, Scripts and Style Sheets

3. After you've finished don't forget to press the Save Configuration button.

Magento 2 Required Properties, Save Configuration

4. Flush the cache and you are free to test it. However, it may take some time for Facebook to update its cache.